Exhibit 99.6

JOINDER TO STOCKHOLDERS’ AGREEMENT

This Joinder Agreement (this “Joinder Agreement”) is made as of the date written below by the undersigned (the “Joining Party”) in accordance with the Stockholders’ Agreement dated as of May 10, 2017 (the “Stockholders’ Agreement”) by and among Syneos Health, Inc. (f/k/a INC Research Holdings, Inc.), Thomas H. Lee Equity Fund VI, L.P. and the other signatories that are party thereto as the same may be amended from time to time.  Capitalized terms used, but not defined, herein shall have the meaning ascribed to such terms in the Stockholders’ Agreement.

The Joining Party hereby acknowledges, agrees and confirms that, by its execution of this Joinder Agreement, the Joining Party shall be deemed to be a party to the Stockholders’ Agreement as of the date hereof and shall have all of the rights and obligations of a Stockholder as if it had executed the Stockholders’ Agreement.  The Joining Party hereby ratifies, as of the date hereof, and agrees to be bound by, all of the terms, provisions and conditions contained in the Stockholders’ Agreement.

IN WITNESS WHEREOF, the undersigned has executed this Joinder Agreement as of the date written below.

Date:
June 24, 2019
	THL Fund VI (2019) Coinvestment Partners, L.P.
	By: Thomas H. Lee Partners, L.P., its sole member
	By: Thomas H. Lee Advisors, LLC, its general partner
	By: THL Holdco, LLC, its managing member
	By:	/s/ Charles P. Holden
	Name:	Charles P. Holden
	Title:	Managing Director
	Address for Notices:
	THL Fund VI (2019) Coinvestment Partners, L.P.
	c/o Thomas H. Lee Partners, L.P.
	100 Federal Street, 35th Floor
	Boston, Massachusetts 02110
	Attention: General Counsel
	Fax: 617-227-3514
	E‑mail: SWolkon@THL.com
	With copies to:
	Ropes & Gray LLP
	Prudential Tower
	800 Boylston Street
	Boston, MA 02199-3600
	Attention: William E. Mone, Esq.
	Facsimile: (617) 951-7050
	E‑mail: william.mone@ropesgray.com