Sebok, the former managing director of BBDO in New York, succeeds Chris Jacques, who quit in January last year after just three months in the job.
He will oversee 1,800 staff at six Y&R offices in the US and Canada. He will be based in New York and starts his new job early next month.
Hamish McLennan, Y&R's worldwide chief executive, described Sebok as "an experienced and inspired client leader", who had worked across "an incredible range of industries and brands".
Sebok, who was at Y&R from 1987 to 1992, has worked with clients including PepsiCo, FedEx, Pizza Hut and Motorola.