UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No.             )*

KCG Holdings, Inc.

(Name of Issuer)

Class A Common Stock, par value $0.01 per share

(Title of Class of Securities)

48244B 100

(CUSIP Number)

John R. Flynn

Wicklow Capital, Inc.

53 W. Jackson Boulevard, Suite 1204

Chicago, Illinois

(312) 360-1377

with a copy to:

Frederick C. Lowinger

Sidley Austin LLP

One South Dearborn Street

Chicago, Illinois 60603

(312) 853-7238

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

July 1, 2013

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box. ¨

*The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 48244B 100

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CUSIP No. 48244B 100

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CUSIP No. 48244B 100

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CUSIP No. 48244B 100

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This statement on Schedule 13D (this “Statement”) relates to the Class A common stock, par value $0.01 per share (the “Class A Common Shares”), of KCG Holdings, Inc., a Delaware corporation (“KCG Holdings”). The principal executive offices of KCG Holdings are located at 545 Washington Boulevard, Jersey City, New Jersey 07310.

Each Reporting Person may be deemed to have acquired beneficial ownership of 18,456,466 Class A Common Shares (inclusive of the Warrant Shares) in connection with the transactions contemplated by the Merger Agreement (as defined below).

In connection with the Amended and Restated Agreement and Plan of Merger, dated as of December 19, 2012 and amended and restated as of April 15, 2013, by and among GETCO Holding Company, LLC (“GETCO”), GA-GTCO, LLC (“GA-GTCO”), Knight Capital Group, Inc. (“Knight”), Knight Holdco, Inc. (now KCG Holdings), Knight Acquisition Corp, GETCO Acquisition, LLC and GA-GTCO Acquisition, LLC (the “Merger Agreement”), which is attached hereto as Exhibit 99.1, (i) Knight Acquisition Corp, a wholly owned direct subsidiary of KCG Holdings, merged with and into Knight, with Knight surviving the merger (the “Knight Merger”), (ii) GETCO Acquisition, LLC, a wholly owned direct subsidiary of KCG Holding, merged with and into GETCO, with GETCO

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surviving the merger (the “GETCO Merger”) and (iii) GA-GTCO merged with and into GA-GTCO Acquisition, LLC, a wholly owned direct subsidiary of KCG Holdings, with GA-GTCO Acquisition, LLC surviving the merger (the “GA-GTCO Merger” and, together with the Knight Merger and the GETCO Merger, the “Mergers”). Upon the consummation of the Mergers on July 1, 2013, and in exchange for the 2,517,684 GETCO units it held prior to the Mergers, the Daniel V. Tierney 2011 Trust received 13,743,517 Class A Common Shares and Class A Warrants, Class B Warrants and Class C Warrants representing the right to purchase 4,712,949 Warrant Shares.

The Daniel V. Tierney 2011 Trust acquired the Class A Common Shares and Warrant Shares reported on this Statement pursuant to the Mergers. Daniel V. Tierney is a director of KCG Holdings and in such capacity participates in the decisions made by the Board of Directors of KCG Holdings in the ordinary course of business of KCG Holdings.

The Reporting Persons do not have any present plans or proposals which relate to or would result in: (i) any acquisition by any person of additional securities of KCG Holdings, or any disposition of securities of KCG Holdings; (ii) any extraordinary corporate transaction, such as a merger, reorganization or liquidation, involving KCG Holdings or any of its subsidiaries; (iii) any sale or transfer of a material amount of assets of KCG Holdings or any of its subsidiaries; (iv) any change in the present board of directors or management of KCG Holdings, including any plans or proposals to change the number or term of directors or to fill any existing vacancies on the board; (v) any material change in the present capitalization or dividend policy of KCG Holdings; (vi) any other material change in KCG Holdings’ business or corporate structure; (vii) any change in KCG Holdings’ charter or bylaws or other actions which may impede the acquisition of control of KCG Holdings by any person; (viii) any delisting from a national securities exchange or any loss of authorization for quotation in an inter-dealer quotation system of a registered national securities association of a class of securities of KCG Holdings; (ix) any termination of registration pursuant to Section 12(g)(4) of the Exchange Act of a class of equity securities of KCG Holdings; or (x) any action similar to any of those enumerated above.

Notwithstanding the foregoing, the Reporting Persons may determine to change their intentions with respect to KCG Holdings at any time in the future and may, for example, elect (i) to acquire additional Class A Common Shares or rights to acquire Class A Common Shares in open market or privately negotiated transactions or, with respect to Daniel V. Tierney, under plans of KCG Holdings in which non-employee directors of KCG Holdings participate, or (ii) to dispose of all or a portion of their holdings of Class A Common Shares or Warrant Shares. In reaching any determination as to a future course of action, the Reporting Persons will take into consideration various factors, such as KCG Holdings’ business and prospects, other developments concerning KCG Holdings, other business opportunities available to the Reporting Persons, estate planning considerations and general economic and stock market conditions, including, but not limited to, the market price of the Class A Common Shares.

(a) – (b) This Statement relates to the beneficial ownership of 18,456,466 Class A Common Shares, which include (i) 13,743,517 Class A Common Shares and (ii) 4,712,949 Warrant Shares. Such shares represent 15.1% of the outstanding Class A Common Shares (based on a total of 122,412,949 Class A Common Shares outstanding, which consists of (i) an estimated 117,700,000 Class A Common Shares outstanding as of July 1, 2013, according to information filed by KCG Holdings on July 1, 2013 and (ii) 4,712,949 Warrant Shares). All of the Class A Common Shares and Warrant Shares reported on this Schedule 13D are directly held by the Daniel V. Tierney 2011 Trust. Emma Cuadrado and John R. Flynn are the co-trustees and share voting and dispositive power over the securities held by the trust. Daniel V. Tierney is the settlor and sole beneficiary of the trust. Daniel V. Tierney does not have or share voting or dispositive power over the securities held by the trust, but does have the power to revoke the trust and acquire beneficial ownership of such securities within 60 days.

Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission by Daniel V. Tierney, Emma Cuadrado or John R. Flynn that such Reporting Person is the beneficial owner of any of the securities referred to herein for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended, or for any other purpose, and such beneficial ownership is expressly disclaimed.

(c) As described in Item 3 of this Statement, the Daniel V. Tierney 2011 Trust acquired beneficial ownership of the Class A Common Shares and Warrant Shares pursuant to the Merger Agreement on July 1, 2013. The Daniel V. Tierney 2011 Trust has entered into a Registration Rights Agreement with KCG Holdings within the last 60 days,

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pursuant to which, among other things, KCG Holdings filed with the Securities and Exchange Commission a shelf registration statement on Form S-3 covering resales of the Class A Common Shares and Warrant Shares received by the Daniel V. Tierney 2011 Trust in the Mergers. No Reporting Person has effected any other transaction in the Class A Common Shares or Warrant Shares during the past 60 days.

(d) To the knowledge of the Reporting Persons, no other person has the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the Class A Common Shares or Warrant Shares covered by this Statement.

(e) Not applicable.

Except as set forth in Item 5 of this Statement, no Reporting Person has any contracts, arrangements, understandings or relationships with respect to any securities of KCG Holdings.

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SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this Statement is true, complete and correct.

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INDEX OF EXHIBITS

Exhibit 99.3

JOINT FILING AGREEMENT

The undersigned hereby agree that the Statement on Schedule 13D, dated July 11, 2013 (the “Schedule 13D”), with respect to the Class A common stock, par value $0.01 per share, of KCG Holdings, Inc. is, and any amendments executed by us shall be, filed on behalf of each of us pursuant to and in accordance with the provisions of Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, and that this Agreement shall be included as an exhibit to the Schedule 13D and each such amendment. Each of the undersigned agrees to be responsible for the timely filing of the Schedule 13D and any amendments thereto, and for the completeness and accuracy of the information concerning itself contained therein. This Agreement may be executed in any number of counterparts, all of which taken together shall constitute one and the same instrument.

IN WITNESS WHEREOF, the undersigned have executed this Agreement as of the dates written below.