He joins from Mustoes, where he held the position of executive creative director.
The BBH executive creative director, John O'Keeffe, said: "Mick comes with a great pedigree both as a creative, and as a creative director. We're very pleased to have him aboard."
Mahoney joined the then Mustoe Merriman Levy in 2001 with his partner, Andy Amadeo. The pair were formerly joint creative directors at CDP and left when the agency announced its merger with travissully.
Mahoney is the latest in a number of creative signings at BBH. Earlier this month, O'Keeffe hired Nick Allsop and Simon Veksner, the team behind DDB London's "cops" ad for Volkswagen Polo.