Chris Willingham has been running the account at TBWA/London for the past six years, at one time in partnership with Garry Lace, now Grey's chief executive.
Industry sources say there is no immediate prospect of the business leaving TBWA but that Grey is keen to put down its marker should the account ever be reviewed.
Willingham, 36, becomes one of Grey's managing partners, working under the senior team running the agency.
The new management structure follows Lace's decision to dissolve the agency's 25-member board. "I want a functioning board at the top of the company," Lace said.
Control of the agency is now in the hands of Lace, the executive creative director, Dave Alberts, the planning chief, Dylan Williams, the new-business director, Nicola Mendelsohn, and the managing director, Chris Hirst.
Willingham will be involved in a number of new-business projects as well as taking on some account responsibilities.
They are likely to include overseeing the £64 million global launch of Nokia's N-Gage mobile-gaming machine, scheduled to make its debut this autumn.
Willingham and Lace first got to know each other when both worked at Saatchi & Saatchi in the early 90s and linked up again when Willingham joined TBWA in 1996. He has also worked on Holsten and News International.
"After seven years with an agency at the top of its game, I wanted a fresh challenge," Willingham said. "What Garry is doing at Grey excites me enormously."
Lace said: "Chris has a track record of producing terrific work as well as being passionate and committed. I am determined to attract the best people and Chris' hiring is a continuation of that policy."