Kay replaces David Golding, who resigned to launch a start-up with James Murphy and Ben Priest. Kay will report to the chief executive, Richard Exon.
Alison Hoad, the RKCR/Y&R vice-chairman, said: "We talked to many people about the role, but I just kept thinking, 'we have the best person for the job here'."
Kay was a founding partner of the RKCR/Y&R integrated offshoot Swarm. Before that he was the deputy planning director for three years. Kay moved to the agency from Young & Rubicam in New York, and has worked at WPP offices in Singapore and Toronto.
Exon said: "Agencies often lose people because there's no vacancy at the top, so being able to refresh the management team with talent like Ben is great news for us."
Kay added: "My job is to harness, build and diversify talent, creating the most effective campaigns in the country. I can't think of a more exciting job in the industry."
Meanwhile, RKCR/Y&R has promoted Jon Tipple to the position of deputy head of planning.