Exhibit 24

         LIMITED POWER OF ATTORNEY FOR SECTION 16 REPORTING OBLIGATIONS

      The undersigned hereby constitutes and appoints each of John McCarthy,
Robert McQueen, Alex Sadowski and Mallory McFarland, signing jointly or any of
them acting singly and in his or her capacity hereunder, with full power of
substitution or resubstitution, as the undersigned's true and lawful attorney-
in-fact to:

    1.   prepare, execute in the undersigned's name and on the undersigned's
behalf and submit to the U.S. Securities and Exchange Commission (the "SEC") a
Form ID, including amendments thereto, and any other documents necessary or
appropriate to obtain codes and passwords enabling the undersigned to make
electronic filings with the SEC of reports required by Section 16(a) of the
Securities and Exchange Act of 1934, as amended (the "Exchange Act") or any rule
or regulation of the SEC;

    2.   execute for and on behalf of the undersigned, in the undersigned's
capacity as an officer and/or director of KCG Holdings, Inc. (the "Company"),
Forms 3, 4 and 5 (and any amendments thereto) in accordance with Section 16(a)
of the Exchange Act, and the rules thereunder;

    3.   do and perform any and all acts for and on behalf of the undersigned
which may be necessary or desirable to complete and execute any such Form 3, 4
or 5, complete and execute any amendments thereto, and timely file such form
with the SEC and any securities exchange or similar authority; and

    4.   take any other action in connection with the foregoing which, in the
opinion of such attorney-in-fact, may be of benefit to, in the best interest of,
or legally required by or for, the undersigned, it being understood that the
documents executed by such attorney-in- fact on behalf of the undersigned
pursuant to this Limited Power of Attorney shall be in such form and shall
contain such information and disclosure as such attorney- in-fact may approve in
such attorney-in-fact's discretion.

      The undersigned hereby grants to each such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever required,
necessary or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's substitute or substitutes, shall lawfully do or cause to be
done by virtue of this Limited Power of Attorney and the rights and powers
herein granted.  The undersigned acknowledges that the foregoing attorneys-in-
fact, in serving in such capacity at the request and on the behalf of the
undersigned, are not assuming, nor is the Company assuming, any of the
undersigned's responsibilities to comply with, or any liability for the failure
to comply with, any provision of Section 16 of the Exchange Act.

      This Limited Power of Attorney shall remain in full force and effect until
the undersigned is either no longer a no longer a member of the Board of
Directors of the Company, employed by the Company or required to file Forms 3, 4
or 5 with respect to the undersigned's holdings of and transactions in
securities issued by the Company, unless earlier revoked by the undersigned in a
signed writing delivered to each of the foregoing attorneys-in-fact.

IN WITNESS WHEREOF, the undersigned has executed this Limited Power of Attorney
as of this 22nd day of September, 2014.



                                           Signed and acknowledged:


                                           By:    /s/ Philip Allison
                                                  -----------------------------
                                                  Name: Philip Allison