Brunier will be responsible for the Leo Burnett Group in Paris, which includes the ad agency Leo Burnett Paris, Leo B to B, Leo Events, Leo Corporate, Arc, Challenger House and Media Book.
He will work alongside Mercier over the coming weeks with both the agency's clients and personnel to ensure a smooth handover.
Brunier joins from Publicis Worldwide where he was responsible for building the global Renault account for the network over the past five years.
He joined Publicis Conseil in 2000, taking on the role of worldwide account director in 2003 and also becoming a member of the agency's management board.
Prior to that, Brunier worked at FCB Worldwide Brussels, Hachette Filippachi/Tele 7 Jours and McCann Erickson France.
In his new role, Brunier will become a member of Leo Burnett's global leadership council and will report to Giorgio Brenna, chairman and CEO of the agency's continental Western Europe region.
Tom Bernardin, chairman and CEO of Leo Burnett Worldwide, said: "France is an extremely important market for our global network and is and will remain a strategic priority for Burnett.
"Brunier joins us at an incredibly exciting time and is exactly the right type of person to carry us forward. I wish Patrick, whose entrepreneurial spirit I admire hugely, nothing but the best in his next challenges."