Millar will lead ZenithOptimedia and its group brands Performics (digital performance marketing), Newcast (branded content) and Ninah (analytics).
He will report to Belinda Rowe, the global managing partner and UK chairman of ZenithOptimedia, who has been helping to run the agency since Tim Hipperson left after just six months as the chief executive in November last year.
At Dentsu Aegis Network, Millar was the president of global clients, responsible for accounts such as Mondelez International and MasterCard.
Rowe said: "Grant Millar is one of the key figures in the UK media industry and we are absolutely thrilled that he is now going to lead ZenithOptimedia in the UK. He has an amazing track record, with proven results in driving results for clients, new business and agency development."
Millar added: "ZenithOptimedia UK has an impressive array of capabilities and an equally impressive array of clients. I am delighted to be joining the agency at a time when it is breaking new ground in its strategic approach and integrated offering for brands."
Millar is a former global brand director at Carat and managing director of Vizeum UK.