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): August 13, 2015

Farmers National Banc Corp.

(Exact name of registrant as specified in its charter)

(330) 533-3341

(Registrant’s telephone number, including area code)

(Former name or former address, if changed since last report)

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 August 13, 2015, Farmers National Banc Corp. (the “Company”) first utilized an investor presentation reflecting the Company’s financial results for the quarter ended June 30, 2015. The investor presentation also reviewed the Company’s previously announced proposed merger with Tri-State 1^st Banc, Inc. (“Tri-State”), the parent company of 1^st National Community Bank (“FNCB”), pursuant to the Agreement and Plan of Merger (the “Merger Agreement”) among the Company, Tri-State, and FMNB Merger Subsidiary, LLC, a newly-formed wholly-owned subsidiary of the Company (the “Merger”).

A copy of the investor presentation is attached hereto as Exhibit 99.1 and incorporated herein by reference. The investor presentation is furnished herein, as part of this Item 7.01, as Exhibit 99.1. Pursuant to General Instruction B.2 of Current Report on Form 8-K, the information in this Item 7.01 and Exhibit 99.1 shall not be deemed to be “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liability of that section. Furthermore, the information in this Item 7.01 and Exhibit 99.1 shall not be deemed to be incorporated by reference into the filings of the Company under the Securities Act of 1933, as amended (the “Securities Act”), or the Exchange Act, except as may be expressly set forth by specific reference in such filing.

On August 13, 2015, the Company first utilized an investor presentation reflecting the Company’s financial results for the quarter ended June 30, 2015, and reviewing the Merger.

Important Additional Information About the Merger.

In connection with the proposed Merger, the Company has filed with the Securities and Exchange Commission (the “SEC”) a Registration Statement on Form S-4 includes a Tri-State proxy statement and a Company prospectus, as well as other relevant documents concerning the proposed transaction.

SHAREHOLDERS OF TRI-STATE AND OTHER INVESTORS ARE URGED TO CAREFULLY READ THE PROXY STATEMENT/PROSPECTUS TO BE INCLUDED IN THE FINAL EFFECTIVE REGISTRATION STATEMENT ON FORM S-4, BECAUSE IT WILL CONTAIN IMPORTANT INFORMATION ABOUT THE COMPANY, TRI-STATE, THE PROPOSED MERGER, THE PERSONS SOLICITING PROXIES WITH RESPECT TO THE PROPOSED MERGER AND THEIR INTERESTS IN THE PROPOSED MERGER AND RELATED MATTERS.

The respective directors and executive officers of the Company and Tri-State and other persons may be deemed to be participants in the solicitation of proxies from shareholders of Tri-State with respect to the proposed Merger. Information regarding the directors and executive officers of the Company is available in its proxy statement filed with the SEC on March 13, 2015. Information regarding directors and executive officers of Tri-State is available on its website at http://www.1stncb.com/InvestorRelations.htm. Other information regarding the

participants in the solicitation and a description of their direct and indirect interests, by security holdings or otherwise, will be contained in the proxy statement/prospectus included in the Registration Statement on Form S-4 and other relevant materials filed with the SEC when they become available.

Investors and security holders will be able to obtain free copies of the registration statement and other documents filed with the SEC by Farmers through the website maintained by the SEC at http://www.sec.gov. Copies of the documents filed with the SEC by Farmers will be available free of charge on Farmers’ website at https://www.farmersbankgroup.com.

This communication shall not constitute an offer to sell or the solicitation of an offer to buy any securities nor shall there be any sale of securities in any jurisdiction in which the offer, solicitation or sale is unlawful before registration or qualification of the securities under the securities laws of the jurisdiction. No offer of securities shall be made except by means of a prospectus satisfying the requirements of Section 10 of the Securities Act.

Forward Looking Statements.

This report contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These statements are not historical facts, but rather statements based on the Company’s current expectations regarding its business strategies and its intended results and future performance. Forward-looking statements are preceded by terms such as “expects,” “believes,” “anticipates,” “intends” and similar expressions, as well as any statements related to future expectations of performance or conditional verbs, such as “will,” “would,” “should,” “could” or “may.”

Forward-looking statements are not guarantees of future performance. Numerous risks and uncertainties could cause or contribute to the Company’s actual results, performance, and achievements to be materially different from those expressed or implied by the forward-looking statements. Factors that may cause or contribute to these differences include, without limitation, the Company’s failure to integrate Tri-State and its subsidiary in accordance with expectations; deviations from performance expectations related to Tri-State and its subsidiary; general economic conditions, including changes in market interest rates and changes in monetary and fiscal policies of the federal government; legislative and regulatory changes; competitive conditions in the banking markets served by the Company’s subsidiaries; the adequacy of the allowance for losses on loans and the level of future provisions for losses on loans; and other factors disclosed periodically in the Company’s filings with the SEC.

Because of the risks and uncertainties inherent in forward-looking statements, readers are cautioned not to place undue reliance on them, whether included in this report or made elsewhere from time to time by the Company or on the Company’s behalf. The Company assumes no obligation to update any forward-looking statements.

The following exhibit is filed herewith:

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Date: August 13, 2015