JOINDER TO STOCKHOLDERS’ AGREEMENT

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.), Xxxxxx X. Xxx 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: Xxxxxx X. Xxx Partners, L.P., its sole member
	By: Xxxxxx X. Xxx Advisors, LLC, its general partner
	By: THL Holdco, LLC, its managing member
	By:	/s/ Xxxxxxx X. Xxxxxx
	Name:	Xxxxxxx X. Xxxxxx
	Title:	Managing Director
	Address for Notices:
	THL Fund VI (2019) Coinvestment Partners, L.P.
	c/o Xxxxxx X. Xxx Partners, L.P.
	000 Xxxxxxx Xxxxxx, 00xx Xxxxx
	Xxxxxx, Xxxxxxxxxxxxx 00000
	Attention: General Counsel
	Fax: 000-000-0000
	E‑mail: XXxxxxx@XXX.xxx
	With copies to:
	Ropes & Xxxx LLP
	Prudential Tower
	000 Xxxxxxxx Xxxxxx
	Xxxxxx, XX 00000-0000
	Attention: Xxxxxxx X. Xxxx, Esq.
	Facsimile: (000) 000-0000
	E‑mail: xxxxxxx.xxxx@xxxxxxxxx.xxx