Exhibit 5.1

Simpson Thacher & Bartlett LLP

425 LEXINGTON AVENUE

NEW YORK, NY 10017-3954

TELEPHONE: +1-212-455-2000

FACSIMILE: +1-212-455-2502

August 21, 2020

Avantor, Inc.

Radnor Corporate Center

Building One, Suite 200

100 Matsonford Road

Radnor, PA 19087

Ladies and Gentlemen:

We have acted as counsel to Avantor, Inc., a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-3 (File No. 333-248127) (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to the sale from time to time of shares of common stock, par value $0.01 per share (the “Common Stock”), of the Company by certain selling stockholders. This opinion relates to the sale by the selling stockholders referred to in the Prospectus (as defined below) (the “Selling Stockholders”) of an aggregate of 63,888,888 shares of Common Stock (the “Shares”) pursuant to an Underwriting Agreement, dated August 19, 2020 (the “Underwriting Agreement”), among the Company, the Selling Stockholders and the underwriters named therein.

We have examined the Registration Statement; the prospectus, dated August 19, 2020 (the “Base Prospectus”), as supplemented by the prospectus supplement dated August 19, 2020 relating to the Shares (together with the Base Prospectus, the “Prospectus”), filed by the Company pursuant to Rule 424(b) of the rules and regulations of the Commission under the Securities Act;

and the Underwriting Agreement. In addition, we have examined, and have relied as to matters of fact upon, originals, or duplicates or certified or conformed copies, of such 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 other investigations as we have deemed relevant and necessary in connection with the opinions hereinafter set forth.

In rendering the opinion set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies and the authenticity of the originals of such latter documents.

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that the Shares have been validly issued and are fully paid and nonassessable.

We do not express any opinion herein concerning any law other than the law of the State of New York and the Delaware General Corporation Law.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Current Report on Form 8-K filed by the Company with the Commission on August 21, 2020 and to the use of our name under the caption “Legal Matters” in the Prospectus.

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