The last person to fill the role at BBH was Gwyn Jones, now the group chief operating officer, who vacated the role in 2005 to become the chief executive of the micro-network's US office.
Charlie Rudd, the deputy managing director, has been promoted to replace Fennell as the managing director.
Fennell, who moved back to BBH's London office in 2006 following a four-year stint as the chief executive of BBH Asia-Pacific, has been effectively running the agency since his return, but will now take sole control of the London office.
In the past two years, he has pushed BBH into non- traditional advertising media including digital, mobile and branded content as well as expanding its board to include more creatives.
Rudd, who has been the deputy managing director since 2006, takes over the day-to-day running of the business, leaving Fennell to concentrate on the agency's growth.
Fennell joined BBH in 1994 as a graduate. After working on a number of accounts, he joined the board in 2004. Rudd joined the agency in 1998 and held various account positions, including three years as the head of account management. He also led the successful global Persil pitch in 2005.