The deal is a result of long-running negotiations over the future of Zenith Media, the media-buying network jointly owned by Publicis and Cordiant.
Fittingly, the announcement of the deal comes on the same day that rival French advertising group Havas bids to take control of the UK's Tempus Group, which owns the CIA media-buying network.
Under the agreement, Publicis-owned Optimedia will be combined with Zenith to create a new company called NewCo. This will be 75% owned by the French advertising group and 25% by Cordiant.
Cordiant CEO Michael Bungey said, "We have exhaustively examined the options for Zenith and I am sure that this is the best solution for our shareholders, our clients and Zenith's people."
Last year, Optimedia posted revenues of £55.9m and pre-tax profits of £12.6m. The agency was created in 1998 and its clients include British Airways, Nestle and L'Oreal and it has won new business from Vizzavi, Ralph Lauren Polo, Kirch and Honda.
Zenith recorded £67.5m in revenues and pre-tax profits of £13.7m. Its clients include Toyota/Lexus, Mars, BT and HSBC with new business wins including General Mills and United Airlines.
Zenith was formerly owned by Cordiant and Saatchi & Saatchi. Publicis gained its 50% stake when it acquired Saatchis last summer.