Exhibit 5.1

Brent B. Siler
(202) 728-7040
bsiler@cooley.com
May 8, 2015
Millennial Media, Inc.
2400 Boston Street, Suite 300
Baltimore, Maryland 21224
Ladies and Gentlemen:
We have represented Millennial Media, Inc., a Delaware corporation (the “Company”) in connection with the filing by the Company of a Registration Statement on Form S-8 (the “Registration Statement”) with the Securities and Exchange Commission covering the offering of up to 8,275,095 shares of the Company’s common stock, par value $0.001 per share (the “Shares”), pursuant to the Company’s 2012 Equity Incentive Plan (the “Plan”).
In connection with this opinion, we have examined and relied upon (a) the Registration Statement and the related prospectus, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, each as currently in effect, (c) the Plan and (d) the originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness and authenticity of all documents submitted to us as originals, and the conformity to originals of all documents submitted to us as copies thereof (except that such assumption is not made with respect to the due execution and delivery of documents by the Company). As to certain factual matters, we have relied upon a certificate of officers of the Company and have not sought to independently verify such matters.
Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware. We express no opinion as to whether the laws of any particular jurisdiction other than those identified above are applicable to the subject matter of this opinion. We are not rendering any opinion as to compliance with any federal or state antifraud law, rule or regulation relating to securities, or to the sale or issuance thereof.
On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued in accordance with the Plan, and the Registration Statement and related prospectus, will be validly issued, fully paid and nonassessable (except as to shares issued pursuant to certain deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments are made in full).
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