Exhibit 5.1

October 27, 2014

INC Research Holdings, Inc.

3201 Beechleaf Court, Suite 600

Raleigh, North Carolina 27604-1547

Ladies and Gentlemen:

We have acted as counsel to INC Research Holdings, Inc., a Delaware corporation (the “Company”), in connection with the preparation and filing with the Securities and Exchange Commission of the Company’s Registration Statement on Form S-1 (Registration Statement No. 333-199178)  (as amended, the “Registration Statement”), under the Securities Act of 1933, as amended (the “Act”), relating to the registration of the offer, issuance and sale by the Company of the number of shares of Class A common stock, par value $0.01 per share (the “common stock”), of the Company specified in the Registration Statement (together with any additional shares that may be sold by the Company pursuant to Rule 462(b) under the Act, the “Shares”). The Shares are to be sold by the Company pursuant to an underwriting agreement among the Company and the underwriters named therein (the “Agreement”), the form of which has been filed as Exhibit 1.1 to the Registration Statement.

In so acting, we have examined originals or copies (certified or otherwise identified to our satisfaction) of (i) the Amended and Restated Certificate of Incorporation of the Company, the form of which has been filed as Exhibit 3.1 to the Registration Statement; (ii) the Amended and Restated Bylaws of the Company, the form of which has been filed as Exhibit 3.2 to the Registration Statement; (iii) the Registration Statement; (iv) the prospectus contained within the Registration Statement; (v) the form of the Agreement; (vi) the form of common stock certificate of the Company which is filed as Exhibit 4.1 to the Registration Statement; and (vii) such corporate records, agreements, documents and other instruments, and such certificates or comparable documents of public officials and of officers and representatives of the Company, and have made such inquiries of such officers and representatives, as we have deemed relevant and necessary as a basis for the opinion hereinafter set forth.

In such examination, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified, conformed or photostatic copies, and the authenticity of the originals of such latter documents. As to all questions of fact material to this opinion that have not been independently established, we have relied upon certificates or comparable documents of officers and representatives of the Company.

Based on the foregoing, and subject to the qualifications stated herein, we are of the opinion that the Shares, when issued and sold as contemplated in the Registration Statement and the Agreement, and upon

payment and delivery in accordance with the Agreement, will be validly issued, fully paid and non-assessable.

The opinions expressed herein are limited to the corporate laws of the State of Delaware and we express no opinion as to the effect on the matters covered by this letter of the laws of any other jurisdiction.

We hereby consent to the filing of this letter as an exhibit to the Registration Statement, to the incorporation by reference of this letter into any subsequent registration statement on Form S-1 filed by the Company pursuant to Rule 462(b) of the Act with respect to the Shares and to the reference to our firm under the caption “Legal Matters” in the prospectus which is a part of the Registration Statement. In giving such consent we do not hereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Securities and Exchange Commission.

Very truly yours,

/s/ Weil, Gotshal & Manges