W&K is in the process of finding a replacement for Smith and has drawn up a shortlist of potential candidates for the role.
Smith joined W&K as a senior planner in 2003, before becoming the head of planning following the departure of the agency's former planning chief, Russell Davies.
During his five-year stint at the agency, Smith has worked across a number of W&K's key accounts, including Honda and Nike, as well as leading the planning on the Nokia handset pitch, which the agency secured in July of last year.
In his new role, Smith will be leading the strategic offering at Google's new creative agency, Google Creative lab, which will produce content in order to promote the Google brand.
Smith said: "I can't quite believe how lucky I've been to be at W&K over the last five years. Everything about it has been extraordinary - the place, the philosophy, the work and, above all, the people. There's no way I would ever have left to join another agency. But the chance to start something small and new, within Google, in New York, was just a little too irresistible."