It is the second high-profile departure in the space of a month at Leo Burnett, which was acquired by Publicis Groupe when it bought Bcom3 in September 2001.
Brennan's exit follows that in September of Steve Gatfield, who stepped down from his role as global chief operating officer role at the agency, although he remains as a consultant until the end of the year.
It has not been a sterling new-business year for Leo Burnett, which has lost several key accounts, including Philips Electronics, Delta Air Lines and Polaroid. However, it has picked up work from PC giant Gateway, as well as US food giant ConAgra.
Brennan's responsibilities will be taken up in the short term by Linda Wolf, chief executive of the agency, while Roger Haupt, president and chief operating officer at Publicis Groupe, will help oversee Leo Burnett's Europe, Middle East and Africa operation while a regional managing director is sought.
"As we determine the best management structure for our global network and our multinational clients, it's important to know that both Roger and I are stepping in to focus on and assist in two key regions -- the US and Europe," Wolf said.
Brennan, 44, has been part of Leo Burnett for 21 years, firstly at Starcom MediaVest, where he worked his way up to the role of chief operating officer with a strong reputation for winning new business, before taking up the role of president of Leo Burnett in 2000. His plans following his resignation have not been revealed.
Leo Burnett is one of three advertising networks at the group, along with Publicis, Saatchi & Saatchi. D'Arcy, the fourth agency network owned by Publicis following its Bcom3 acquisition, was closed last year.
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