Chalk is returning to the UK after seven years in the US. He was most recently the planning partner at Goodby Silverstein & Partners in San Francisco. Before that he was the head of planning at TBWA/Chiat/Day.
At Boymeetsgirl he will hold equal equity with the agency's chairman, Andy Law, its creative director, Kate Stanners, and its chief executive, David Pemsel.
His brief is to build a planning department that creates strategic platforms across all communication disciplines.
Chalk said: "Boymeetsgirl is unique in that culturally it is a start-up. Critically, however, it is not a start-up in terms of resource. There are already 90 people in the agency capable of creating and executing ideas in almost any medium."
Law added: "Chris is the smartest thinker I've met in the last ten years. His unique experience from the States on global integrated accounts such as HP and Saturn Cars have become legendary case histories. We are thrilled to welcome him."