The new venture is the brainchild of Chris King, who has been running Mars' global snacks business out of the London agency, and Andy Blackford, who resigned in March as one of its four creative directors.
The pair said that fully integrated communications will be at the heart of their offering when they open for business in the autumn.
King said the structure of the operation was a response to a growing trend by clients such as COI Communications to demand fully integrated marketing solutions and the increasing willingness by major advertisers to put business into small young agencies.
"Our plan is to be completely integrated from birth," Blackford said.
"Our intention is to have people around us from the first day who are equally comfortable above and below the line."
A former Campaign Face to Watch, King came to Grey in 1998 when the agency took over Mellors Reay, where he was the managing director.
Blackford, a long-time advocate of integrated advertising, has a career stretching back more than 20 years including spells at IMP, Leo Burnett and Aspect and as the creative chief of the Grey-owned integrated specialist Joshua. His most recent work for Grey was on GlaxoSmithKline.