Kirby, 43, will be responsible for the development and implementation of a growth and communications strategy across Saatchis' UK group. He joins from Diageo, where he has been the global advertising procurement director for the past three years.
Chalk, 42, joins the agency from his role as Nokia's global head of brand experience and communications, with a brief to deliver strategic solutions and creative ideas to clients. The pair worked together at Lowe between 1994 and 1997.
They will work closely with Lee Daley, Saatchis' chairman and chief executive, and Kate Stanners, the executive creative director.
Daley said: "I have tracked Kirby for several years and his qualifications are unique in fulfilling this critical role."
He added: "Hiring Chris is a final part of the investment in a world-class board. Chris has vast experience and a brilliant mind."
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