Mantech International (MANT) S-8 Registration of securities for employees

Exhibit 5.1

1650 TYSONS BOULEVARD
SUITE 400
MCLEAN, VIRGINIA 22102   TELEPHONE: 703.760.7700 FACSIMILE: 703.760.7777   WWW.MOFO.COM MORRISON & FOERSTER LLP   NEW YORK, SAN FRANCISCO,
LOS ANGELES, PALO ALTO,
SAN DIEGO, WASHINGTON, D.C.   NORTHERN VIRGINIA, DENVER,
SACRAMENTO, WALNUT CREEK   TOKYO, LONDON, BRUSSELS,
BEIJING, SHANGHAI, HONG KONG

September 28, 2012

ManTech International Corporation

12015 Lee Jackson Highway

Fairfax, Virginia 22033

RE: ManTech International Corporation Management Incentive Plan 2011 Restatement

Ladies and Gentlemen:

At your request, we have examined the Registration Statement on Form S-8 to be filed with the Securities and Exchange Commission in connection with the registration under the Securities Act of 1933, as amended, of an aggregate of 2,983,220 shares (the “Shares”) of the Class A Common Stock, $0.01 par value (the “Common Stock”), of ManTech International Corporation (the “Company”), which will be issuable from time to time under the Management Incentive Plan of ManTech International Corporation 2011 Restatement (the “2011 Plan”).

As your counsel in connection with the Registration Statement, we have examined the proceedings taken by you in connection with the adoption of the Plan and the authorization of the issuance of the Shares, and such documents as we have deemed necessary to render this opinion. For the purpose of the opinion rendered below, we have assumed that in connection with the issuance of the Shares, the Company will receive consideration in an amount not less than the aggregate par value of the Shares covered by each such issuance.

Based upon and subject to the foregoing, it is our opinion that the Shares, when issued and sold pursuant to the terms of the 2011 Plan, as applicable, will be validly issued, fully paid and nonassessable shares of Common Stock. The opinion expressed herein is limited to the General Corporation Law of the State of Delaware, the Delaware Constitution and reported judicial decisions interpreting those laws, each as currently in effect.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement. In giving such consent, we do not hereby admit that we are acting within the category of persons whose consent is required under Section 7 of the Act or the rules or regulations of the Securities and Exchange Commission thereunder.

Very truly yours,

/s/ Morrison & Foerster LLP