UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

(Amendment No. 1)

FOR THE FISCAL YEAR ENDED DECEMBER 31, 2013

— OR —

Commission File Number 1-12833

Energy Future Holdings Corp.

(Exact name of registrant as specified in its charter)

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  ¨    No  x

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports) and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K (229.405 of this chapter) is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  x

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Indicate by check mark if the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x

At April 29, 2014, there were 1,669,861,383 shares of common stock, without par value, outstanding of Energy Future Holdings Corp. (substantially all of which were owned by Texas Energy Future Holdings Limited Partnership, Energy Future Holdings Corp.’s parent holding company, and none of which is publicly traded).

DOCUMENTS INCORPORATED BY REFERENCE

None

EXPLANATORY NOTE

Energy Future Holdings Corp. (the Company) is filing this Amendment No. 1 to the Annual Report on Form 10-K (the Form 10-K/A) to amend its Annual Report on Form 10-K for the year ended December 31, 2013, which was filed with the Securities and Exchange Commission on April 30, 2014 (the Original Filing). The Company is filing this Form 10-K/A solely to provide Exhibits 10(cccc) and 10(dddd), which were not included in the Original Filing.

No other changes have been made to the Original Filing other than the inclusion of Exhibits 10(cccc) and 10(dddd). This Form 10-K/A does not reflect events occurring after the filing date of the Original Filing. The descriptions of the Restructuring Support and Lock-Up Agreement and the Commitment Letter set forth in the Original Filing are subject to, and qualified in their entirety by, the full text of the Restructuring Support and Lock-Up Agreement and Commitment Letter, which are attached hereto as Exhibit 10(cccc) and 10(dddd), respectively, and incorporated herein by reference.

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SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, Energy Future Holdings Corp. has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

EXHIBIT INDEX