Mills first learned guitar before picking up drums when he was ten years old. He played in the swing groups of Charlie Barnet and Boyd Raeburn in the 1940s, then with Jazz at the Philharmonic, Gene Norman, Babe Russin, Manny Klein, Dizzy Gillespie, Benny Goodman, Rene Touzet, Sonny Criss, Andre Previn, Lionel Hampton, Stan Getz, Woody Herman, and Red Norvo. Later in the 1940s he became interested in bebop and began playing in a style influenced by Max Roach. He began playing with Harry James in 1949, working with him through the late 1950s. He recorded as a session musician in the 1950s, including with Gerry Wiggins and Anita O'Day, and did production work for Columbia, MGM, Mainstream, Capitol and Liberty Records, founding the Larrabee Sound Studios. He occasionally recorded in the 1960s, including with Freddie Roach in 1966 and Dodo Marmarosa in 1978, but was chiefly active as a record executive in his later career.