Brad Brinegar, who will remain chairman and CEO of the newly independent McKinney, said that it had been a long-term goal to buy back the agency from Havas, which is chaired by Vincent Bollore.
Havas said that the sale was beneficial for both parties and part of its strategy of focusing on its core global brands.
Brinegar said: "We've enjoyed a very cordial and autonomous working relationship with Havas and are proud of the contributions we have delivered to them.
"But we have long wished to be able to compete as an independent agency, and we were thrilled when this opportunity presented itself."
Fernando Rodes Vila, Havas CEO, said: "We are increasing our focus on our core global brands -- Euro RSCG, Havas Media and Arnold -- so this is a good time to act on the interest Brad and his team have long expressed to own the agency."
The agency's board will be made up of Brinegar and 11 other partners including president Jeff Jones, chief strategy officer Andrew Delbridge, chief operating officer Joni Madison and chief creative officer Jonathan Cude.
McKinney was founded in 1969 and has created campaigns including Audi of America's 'art of the H3ist', Travelocity's 'roaming game', the Polaris 'CEO duel' and the launch of the Sony Bravia LCD TV.