Y&R global CEO and chairman David Sable has been made non-executive chairman but is also taking an as-yet unconfirmed role elsewhere in WPP.
The holding company announced the new agency will be called VMLY&R and will be fully operational in early 2019. It described the agency proposition as being "to create connected brands that drive value for clients".
Cook, who has been global CEO of VML for 22 years, will report to WPP chief executive Mark Read, who said the two agencies "have distinct and complementary strengths spanning creative, technology and data services that make them a perfect match".
"This is an important step as we build a new, simpler WPP that provides clients with a fully integrated offering and easy access to our wealth of talent and resources," Read continued.
Cook said: "I’m thrilled for the VMLY&R team as we start this journey together and harness the best of each agency to deliver culturally relevant world-class work. The landscape of our industry is changing rapidly, and we are committed to being an invaluable partner to CMOs around the world."
VMLY&R clients include Colgate-Palmolive, Danone, Dell, Ford, Office Depot, PepsiCo, Pfizer and Wendy’s.