UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): October 31, 2012

MPLX LP

(Exact name of Registrant as specified in its charter)

200 E. Hardin Street

Findlay, Ohio 45840

(Address of principal executive offices, including zip code)

(419) 672-6500

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On October 31, 2012, MPLX LP (the “Partnership”) completed its initial public offering (the “Offering”) of 19,895,000 common units representing limited partner interests in the Partnership (“Common Units”), which included 2,595,000 Common Units pursuant to the underwriters’ option to purchase additional Common Units, at $22.00 per Common Unit pursuant to a Registration Statement on Form S-1, as amended (File No. 333-182500), initially filed by the Partnership with the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), on July 2, 2012. The material provisions of the Offering are described in the prospectus, dated October 25, 2012, filed with the Commission on October 29, 2012, pursuant to Rule 424(b) under the Securities Act (the “Prospectus”).

Contribution, Conveyance and Assumption Agreement

The description of the Contribution Agreement (as defined below) provided below under Item 2.01 is incorporated in this Item 1.01 by reference. A copy of the Contribution Agreement is filed as Exhibit 10.1 to this Current Report on Form 8-K and incorporated herein by reference.

Omnibus Agreement

On October 31, 2012, in connection with the closing of the Offering, we entered into an Omnibus Agreement (the “Omnibus Agreement”) with our general partner, MPLX GP LLC (the “General Partner”), and Marathon Petroleum Corporation (“MPC”), Marathon Petroleum Company LP (“MPCLP”), MPL Investment LLC (“MPL Investment”), MPLX Operations LLC (the “Operating Company”), MPLX Terminal and Storage LLC (“MTS”), MPLX Pipe Line Holdings LP (“Pipe Line Holdings”), Marathon Pipe Line LLC (“MPL”) and Ohio River Pipe Line LLC (“ORPL”), that addresses the following matters:

So long as MPC controls our general partner, the Omnibus Agreement will remain in full force and effect. If MPC ceases to control our general partner, either party may terminate the Omnibus Agreement, provided that the indemnification obligations will remain in full force and effect in accordance with their terms.

The foregoing description is not complete and is qualified in its entirety by reference to the Omnibus Agreement, which is filed as Exhibit 10.2 to this Current Report on Form 8-K and incorporated herein by reference.

Amended and Restated Operating Agreement

On October 31, 2012, in connection with the closing of the Offering, MPCLP and MPL amended and restated their existing Agreement for Operating Services and entered into an Amended and Restated Operating Agreement (as amended and restated, the “Operating Agreement”) under which MPL will receive an annual $11.2 million operating fee for operating certain MPCLP wholly owned crude oil and product pipeline systems. Under the Operating Agreement, MPCLP will generally reimburse MPL for all direct and indirect costs associated with operating the assets and providing such operational services. The Operating Agreement has an initial term of fourteen months and will automatically renew for additional one-year terms.

The foregoing description is not complete and is qualified in its entirety by reference to the Operating Agreement, which is filed as Exhibit 10.3 to this Current Report on Form 8-K and incorporated herein by reference.

Patoka to Lima Crude System Transportation Services Agreement

On October 31, 2012, in connection with the closing of the Offering, MPL entered into a transportation services agreement (the “Patoka to Lima Crude System Transportation Services Agreement”) with MPCLP under which MPL will charge MPCLP, at the applicable FERC tariff rates, for transporting crude oil from Patoka, Illinois and any new or existing connections, including any injection and truck unloading points, along our Patoka to Lima pipeline system. MPCLP will be obligated to transport on this pipeline system each quarter an average of at least the lesser of: (1) 40 mbpd of light equivalent crude oil and (2) 290 mbpd of light equivalent crude oil minus all third-party shipments of light equivalent crude oil on the system, each quarter on this pipeline system. If MPCLP fails to transport its minimum throughput volumes during any quarter, then MPCLP will pay us a deficiency payment equal to the volume of the deficiency multiplied by the tariff rate then in effect (the “quarterly deficiency payment”). The amount of any quarterly deficiency payment paid by MPCLP may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPCLP’s minimum volume commitment during any of the succeeding four quarters, after which time any unused credits will expire. Upon the expiration or termination of the Patoka to Lima Crude System Transportation Services Agreement, MPCLP will have the opportunity to apply any remaining credit amounts until the completion of the succeeding four quarter period, without regard to the minimum volume commitment under the agreement.

The foregoing description is not complete and is qualified in its entirety by reference to the Patoka to Lima Crude System Transportation Services Agreement, which is filed as Exhibit 10.4 to this Current Report on Form 8-K and incorporated herein by reference.

Catlettsburg and Robinson Crude System Transportation Services Agreement

On October 31, 2012, in connection with the closing of the Offering, MPL entered into a transportation services agreement (the “Catlettsburg and Robinson Crude System Transportation Services Agreement”) with MPCLP under which MPL will charge MPCLP, at the applicable FERC tariff rates, for transporting crude oil from:

MPCLP will be obligated to transport each quarter an average of at least 380 mbpd of light equivalent crude oil from origin points at Patoka, Owensboro or other connections on this pipeline system to MPCLP’s Robinson or Catlettsburg refineries. If MPCLP fails to transport its minimum throughput volumes during any quarter, then MPCLP will pay us a quarterly deficiency payment. The amount of any quarterly deficiency payment paid by MPCLP may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPCLP’s minimum volume commitment during any of the succeeding four quarters, after which time any unused credits will expire. Upon the expiration or termination of the Catlettsburg and Robinson Crude System Transportation Services Agreement, MPCLP will have the opportunity to apply any remaining credit amounts until the completion of the succeeding four quarter period, without regard to the minimum volume commitment under the agreement.

The foregoing description is not complete and is qualified in its entirety by reference to the Catlettsburg and Robinson Crude System Transportation Services Agreement, which is filed as Exhibit 10.5 to this Current Report on Form 8-K and incorporated herein by reference.

Detroit Crude System Transportation Services Agreement

On October 31, 2012, in connection with the closing of the Offering, MPL entered into a transportation services agreement (the “Detroit Crude System Transportation Services Agreement”) with MPCLP under which MPL will charge MPCLP, at the applicable FERC tariff rates, for transporting crude oil from:

MPCLP will be obligated to transport each quarter an average of at least 155 mbpd of light equivalent crude oil from origin points at Samaria, Romulus or other connections on this pipeline system to MPCLP’s Detroit refinery. If MPCLP fails to transport its minimum throughput volumes during any quarter, then MPCLP will pay us a quarterly deficiency payment. The amount of any quarterly deficiency payment paid by MPCLP may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPCLP’s minimum volume commitment during any of the succeeding four quarters, after which time any unused credits will expire. Upon the expiration or termination of the Detroit Crude System Transportation Services Agreement, MPCLP will have the opportunity to apply any remaining credit amounts until the completion of the succeeding four quarter period, without regard to the minimum volume commitment under the agreement.

The foregoing description is not complete and is qualified in its entirety by reference to the Detroit Crude System Transportation Services Agreement, which is filed as Exhibit 10.6 to this Current Report on Form 8-K and incorporated herein by reference.

Wood River to Patoka Crude System Transportation Services Agreement

On October 31, 2012, in connection with the closing of the Offering, MPL entered into a transportation services agreement (the “Wood River to Patoka Crude System Transportation Services Agreement”) with MPCLP under which MPL will charge MPCLP, at the applicable FERC tariff rates, for transporting crude oil from:

MPCLP will be obligated to transport on this pipeline system each quarter an average of at least the lesser of: (1) 130 mbpd of light equivalent crude oil and (2) 279 mbpd light equivalent crude oil minus all third-party shipments of light equivalent crude oil on the system. If MPCLP fails to transport its minimum throughput volumes during any quarter, then MPCLP will pay us a quarterly deficiency payment. The amount of any quarterly deficiency payment paid by MPCLP may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPCLP’s minimum volume commitment during any of the succeeding eight quarters, after which time any unused credits will expire. Upon the expiration or termination of the Wood River to Patoka Crude System Transportation Services Agreement, MPCLP will have the opportunity to apply any remaining credit amounts until the completion of the succeeding eight quarter period, without regard to the minimum volume commitment under the agreement.

The foregoing description is not complete and is qualified in its entirety by reference to the Wood River to Patoka Crude System Transportation Services Agreement, which is filed as Exhibit 10.7 to this Current Report on Form 8-K and incorporated herein by reference.

Garyville Products System Transportation Services Agreement

On October 31, 2012, in connection with the closing of the Offering, MPL entered into a transportation services agreement (the “Garyville Products System Transportation Services Agreement”) with MPCLP under which MPL will charge MPCLP, at the applicable FERC tariff rates, for transporting products from Garyville, Louisiana and any new or existing connections, including any injection and truck unloading points, on our Garyville pipeline system to Baton Rouge and Zachary, Louisiana. MPCLP will be obligated to transport an average each quarter of at least 300 mbpd of products from MPCLP’s Garyville refinery to Baton Rouge or Zachary, and an average each quarter of at least 80 mbpd of products from tankage at Zachary to the Colonial Pipeline in Zachary. Our tariff rates on this pipeline system are market-based. If MPCLP fails to transport its minimum throughput volumes during any quarter, then MPCLP will pay us a quarterly deficiency payment. The amount of any quarterly deficiency payment paid by MPCLP may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPCLP’s minimum volume commitment during any of the succeeding four quarters, after which time any unused credits will expire. Upon the expiration or termination of the Garyville Products System Transportation Services Agreement, MPCLP will have the opportunity to apply any remaining credit amounts until the completion of the succeeding four quarter period, without regard to the minimum volume commitment under the agreement.

The foregoing description is not complete and is qualified in its entirety by reference to the Garyville Products System Transportation Services Agreement, which is filed as Exhibit 10.8 to this Current Report on Form 8-K and incorporated herein by reference.

Texas City Products System Transportation Services Agreement

On October 31, 2012, in connection with the closing of the Offering, MPL entered into a transportation services agreement (the “Texas City Products System Transportation Services Agreement”) with MPCLP under which MPL will charge MPCLP, at the applicable FERC tariff rates, for transporting products on our Texas City pipeline system from Texas City to Pasadena, Texas and from storage tanks at Pasadena to connecting pipeline carriers. MPCLP will be obligated to transport an average each quarter of at least 81 mbpd of products from origin points at Texas City to Pasadena, and an average each quarter of at least 61 mbpd of products from storage tanks at Pasadena to connecting pipeline carriers. Our tariff rates on this pipeline system are market-based. If MPCLP fails to transport its minimum throughput volumes during any quarter, then MPCLP will pay us a quarterly deficiency payment. The amount of any quarterly deficiency payment paid by MPCLP may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPCLP’s minimum volume commitment during any of the succeeding four quarters, after which time any unused credits will expire. Upon the expiration or termination of the Texas City Products System Transportation Services Agreement, MPCLP will have the opportunity to apply any remaining credit amounts until the completion of the succeeding four quarter period, without regard to the minimum volume commitment under the agreement.

The foregoing description is not complete and is qualified in its entirety by reference to the Texas City Products System Transportation Services Agreement, which is filed as Exhibit 10.9 to this Current Report on Form 8-K and incorporated herein by reference.

ORPL Products System Transportation Services Agreement

On October 31, 2012, in connection with the closing of the Offering, ORPL entered into a transportation services agreement (the “ORPL Products System Transportation Services Agreement”) with MPCLP under which ORPL will charge MPCLP, at the applicable FERC tariff rates, for transporting products from:

MPCLP will be obligated to transport an average of at least: 48 mbpd of products each quarter on the Kenova to Columbus pipeline segment; 10 mbpd of products each quarter on the Columbus to Dayton pipeline segment; four mbpd of products each quarter on the Heath to Findlay and Findlay to Heath pipeline segment; six mbpd of products each quarter on the Columbus to Heath or Heath to Columbus pipeline segment; 32 mbpd of products each quarter on the Canton to East Sparta or East Sparta to Canton pipeline segment; eight mbpd of products each quarter on the East Sparta to Heath or Heath to East Sparta pipeline segment; 13 mbpd of products each quarter on the East Sparta to Midland or Midland to East Sparta pipeline segment and seven mbpd of products each quarter on the East Sparta to West Point pipeline segment or Midland to West Point pipeline segment. Once MPCLP has satisfied its minimum throughput commitment on any of the pipeline segments for any quarter, all excess volumes shipped by MPCLP on those segments will be at a reduced incentive tariff rate.

If MPCLP fails to transport its minimum throughput volumes during any quarter, then MPCLP will pay us a quarterly deficiency payment. The amount of any quarterly deficiency payment paid by MPCLP may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPCLP’s minimum volume commitment during any of the succeeding four quarters, after which time any unused credits will expire. Upon the expiration or termination of the ORPL Products System Transportation Services Agreement, MPCLP will have the opportunity to apply any remaining credit amounts until the completion of the succeeding four quarter period, without regard to the minimum volume commitment under the agreement

The foregoing description is not complete and is qualified in its entirety by reference to the ORPL Products System Transportation Services Agreement, which is filed as Exhibit 10.10 to this Current Report on Form 8-K and incorporated herein by reference.

Robinson Products System Transportation Services Agreement

On October 31, 2012, in connection with the closing of the Offering, MPL entered into a transportation services agreement (the “Robinson Products System Transportation Services Agreement”) with MPCLP under which MPL will charge MPCLP, at the applicable FERC tariff rates, for transporting products from:

MPCLP will be obligated to transport an average of at least 209 mbpd of products each quarter in the aggregate from origin points at Robinson, Wood River and Martinsville for volumes delivered from the Dieterich to Martinsville pipeline segment, as well as other connection points on this pipeline system. If MPCLP fails to transport its minimum throughput volumes during any quarter, then MPCLP will pay us a quarterly deficiency payment. The amount of any quarterly deficiency payment paid by MPCLP may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPCLP’s minimum volume commitment during any of the succeeding four quarters, after which time any unused credits will expire. Upon the expiration or termination of the Robinson Products System Transportation Services Agreement, MPCLP will have the opportunity to apply any remaining credit amounts until the completion of the succeeding four quarter period, without regard to the minimum volume commitment under the agreement.

The foregoing description is not complete and is qualified in its entirety by reference to the Robinson Products System Transportation Services Agreement, which is filed as Exhibit 10.11 to this Current Report on Form 8-K and incorporated herein by reference.

Wood River Barge Dock Transportation Services Agreement

On October 31, 2012, in connection with the closing of the Offering, MPL entered into a transportation services agreement (the “Wood River Barge Dock Transportation Services Agreement”) with MPCLP under which MPL will charge MPCLP, at the applicable FERC tariff rates, for transporting crude oil or products over our dock at Wood

River, Illinois to or from barges supplied by MPCLP. MPCLP will be obligated to transport an average of at least the lesser of (1) 40 mbpd of crude oil and products and (2) 60 mbpd of crude oil and products minus all third-party shipments of light equivalent crude oil and products handled each quarter at this facility. If MPCLP fails to transport its minimum throughput volumes during any quarter, then MPCLP will pay us a quarterly deficiency payment. The amount of any quarterly deficiency payment paid by MPCLP may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPCLP’s minimum volume commitment during any of the succeeding eight quarters, after which time any unused credits will expire. Upon the expiration or termination of the Wood River Barge Dock Transportation Services Agreement, MPCLP will have the opportunity to apply any remaining credit amounts until the completion of the succeeding eight quarter period, without regard to the minimum volume commitment under the agreement.

The foregoing description is not complete and is qualified in its entirety by reference to the Wood River Barge Dock Transportation Services Agreement, which is filed as Exhibit 10.12 to this Current Report on Form 8-K and incorporated herein by reference.

Relationships

Each of the Partnership, the General Partner, MPCLP, the Operating Company, MPC Investment LLC, MPLX Logistics Holdings LLC (“Logistics Holdings”), MPL, MPL Investment, MTS, Pipe Line Holdings and ORPL is a direct or indirect subsidiary of MPC. As a result, certain individuals, including officers and directors of MPC and the General Partner, serve as officers and/or directors of more than one of such other entities. As described in Item 2.01 below, the General Partner, as the general partner of the Partnership, holds 1,508,225 general partner units of the Partnership, which represents a 2% general partner interest in the Partnership and MPC, through Logistics Holdings, holds 17,056,515 Common Units and 36,951,515 subordinated units of the Partnership (“Subordinated Units”), which represent an approximate 71.6% limited partner interest in the Partnership.

Contribution, Conveyance and Assumption Agreement

On October 31, 2012, in connection with the closing of the Offering, the Partnership entered into a Contribution, Conveyance and Assumption Agreement (the “Contribution Agreement”) with the General Partner, the Operating Company, MPC Investment LLC, Logistics Holdings, MPL, MPL Investment, Pipe Line Holdings and ORPL. Immediately prior to the closing of the Offering, the following transactions, among others, occurred pursuant to the Contribution Agreement:

These transfers and distributions were made in a series of steps outlined in the Contribution Agreement. The foregoing description is not complete and is qualified in its entirety by reference to the full text of the Contribution Agreement, which is attached as Exhibit 10.1 to this Current Report on Form 8-K and incorporated herein by reference.

The description in Item 2.01 above of the issuance by the Partnership on October 31, 2012 in connection with the consummation of the transactions contemplated by the Contribution Agreement is incorporated in this Item 3.02 by reference. The foregoing transactions were undertaken in reliance upon the exemption from the registration requirements of the Securities Act afforded by Section 4(2) thereof. The Partnership believes that exemptions other than the foregoing exemption may exist for these transactions.

Each of the Subordinated Units granted under the Contribution Agreement will convert into one Common Unit and then will participate pro rata with the other Common Units in distributions of available cash at the end of the subordination period. Unless earlier terminated pursuant to the terms of our Partnership Agreement (as defined below), the subordination period will extend until the first business day of any quarter beginning after December 31, 2015, that the Partnership meets the financial tests set forth in the Partnership Agreement, but may end sooner if the Partnership meets additional financial tests. The description of the subordination period contained in the section entitled “Provisions of our Partnership Agreement Relating to Cash Distributions—Subordinated Units and Subordination Period” of the Prospectus is incorporated herein by reference.

New Directors

Effective with the closing of the Offering on October 31, 2012, David A. Daberko, John P. Surma, Dan D. Sandman and C. Richard Wilson became members of the Board of Directors of our General Partner (the “Board”). Mr. Sandman and Mr. Wilson also became members of the Board’s Audit Committee as well as the Board’s Conflicts Committee. Mr. Wilson will serve as Chair of the Audit Committee.

Effective with the closing of the Offering on October 31, 2012, pursuant to the 2012 MPLX LP Incentive Compensation Plan, Messrs. Daberko, Surma, Sandman and Wilson, as non-employee directors, received a compensation package having an annual value equal to $125,000 and payable as follows:

Effective with the closing of the Offering on October 31, 2012, Messrs. Daberko, Surma, Sandman and Wilson each received a prorated cash payment and a prorated phantom unit award reflecting the period of time remaining in their initial partial quarter of service.

In addition, as the Chair of the Audit Committee, Mr. Wilson will receive an additional annual retainer of $15,000, payable in cash in equal quarterly installments at the commencement of each calendar quarter.

Further, Messrs. Daberko, Surma, Sandman and Wilson will each be indemnified for his actions associated with being a director to the fullest extent permitted under Delaware law and will be reimbursed for all expenses incurred in attending to his duties as a director.

First Amended and Restated Agreement of Limited Partnership of MPLX LP

On October 31, 2012, in connection with the closing of the Offering, the Partnership amended and restated its Agreement of Limited Partnership and entered into the First Amended and Restated Agreement of Limited Partnership of MPLX LP (as amended and restated, the “Partnership Agreement”). A description of the Partnership Agreement is contained in the Prospectus in the section entitled “Our Partnership Agreement” and incorporated herein by reference.

The foregoing description is not complete and is qualified in its entirety by reference to the Partnership Agreement, which is filed as Exhibit 3.1 to this Current Report on Form 8-K and incorporated herein by reference.

Amended and Restated Agreement of Limited Partnership of MPLX Pipe Line Holdings LP

On October 31, 2012, in connection with the closing of the Offering, Pipe Line Holdings amended and restated its Agreement of Limited Partnership and entered into the Amended and Restated Agreement of Limited Partnership of MPLX Pipe Line Holdings LP (as amended and restated, the “Pipe Line Holdings Partnership Agreement”). Pursuant to the Pipe Line Holdings Partnership Agreement, the Operating Company owns a 51% general partner interest in Pipe Line Holdings and MPL Investment owns a 49% limited partner interest in Pipe Line Holdings. Generally, profits and losses will be allocated among the partners of Pipe Line Holdings in proportion to their respective percentage partnership interests. Pipe Line Holdings is required to distribute all distributable cash to its partners on a pro rata basis within 45 days following the end of each quarter. Each partner of Pipe Line Holdings will receive a pro rata portion of distributable cash based on such partner’s respective percentage partnership interests. No partner of Pipe Line Holdings may withdraw capital or receive distributions from Pipe Line Holdings except as provided in the Pipe Line Holdings Partnership Agreement. The management of the business and affairs of Pipe Line Holdings is vested solely in the Operating Company as the general partner of Pipe Line Holdings and no limited partner is entitled to any management power over the business and affairs of Pipe Line Holdings. Notwithstanding the foregoing, the unanimous approval of all partners of Pipe Line Holdings is required for the following:

Third parties will be entitled to rely upon the power and authority of the Operating Company, as the general partner of Pipe Line Holdings, to act on behalf of Holdings. No partner may transfer any part of its partner interests without first obtaining the written approval of the other partners. However, in the event a partner is party to a merger, consolidation, or similar transaction, such partner may transfer all or part of its partner interests to such other person without the approval of the other partners. Except in connection with a permitted transfer, no partner may resign or withdraw as a partner from Pipe Line Holdings without the consent of the other partners.

The foregoing description is not complete and is qualified in its entirety by reference to the Pipe Line Holdings Partnership Agreement, which is filed as Exhibit 3.2 to this Current Report on Form 8-K and incorporated herein by reference.

(d) Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: November 6, 2012

Index to Exhibits