Exhibit 5.1

Babak Yaghmaie
T: 212 479 6556
byaghmaie@cooley.com

October 6, 2017

MongoDB, Inc.

229 W. 43rd Street, 5th Floor

New York, NY 10036

Ladies and Gentlemen:

We have acted as counsel to MongoDB, Inc., a Delaware corporation (the “Company”), in connection with the filing by the Company of a Registration Statement (No. 333-220557) on Form S-1 (the “Registration Statement”) with the U.S. Securities and Exchange Commission, including a related prospectus filed with the Registration Statement (the “Prospectus”), covering an underwritten public offering (the “Offering”) of up to 9,200,000 shares of the Company’s Class A common stock, par value $0.001 per share (“Shares”), including up to 1,200,000 Shares that may be sold by the Company upon exercise of an over-allotment option to be granted to the underwriters.

In connection with this opinion, we have (i) examined and relied upon (a) the Registration Statement and Prospectus, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, as currently in effect as of the date hereof, (c) forms of the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, filed as Exhibits 3.2 and 3.4 to the Registration Statement, respectively, each of which will be in effect upon the closing of the Offering 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 and (ii) assumed that the Shares will be sold at a price and on terms authorized by the Board of Directors of the Company, or the Pricing Committee thereof, in accordance with Section 153 of the General Corporation Law of the State of Delaware (the “DGCL”).

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 and the due execution and delivery of all documents where due execution and delivery are a prerequisite to the effectiveness thereof (except we have not assumed the due execution and delivery by the Company of any such documents). As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not sought independently to verify such matters. Our opinion is expressed only with respect to the DGCL.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued against payment therefore in accordance with the Registration Statement and the Prospectus, will be validly issued, fully paid and non-assessable.

COOLEY LLP 1114 AVENUE OF THE AMERICAS NEW YORK, NY 10036
T: (212) 479-6000 F: (212) 479-6275 COOLEY.COM

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus included in the Registration Statement and to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

COOLEY LLP


By:	/s/ Babak Yaghmaie
	Babak Yaghmaie