Scott will now take full responsibility for the agency's 57-strong account management department.
He replaces Toby Pschorr, who is stepping down from the role but will continue to run a number of accounts.
Scott joined BBH in 2000 following a brief stint at Saatchi & Saatchi. In 2003, he moved to the agency's Singapore office to run the pan-regional Johnnie Walker business. He returned to London in 2005 to run Audi.
Charlie Rudd, the managing director of BBH, said: "Bill represents the very best in BBH account management. He's passionate about client business and our work in equal measure and he's one of the very best team leaders around."