He was the drummer for the seminal Minneapolis-based alternative rock band the Replacements from 1979 to 1990; he later joined the informal supergroup Golden Smog before beginning a solo career.

In Rolling Stone's Alt Rock-a-Rama, Mars detailed the kind of hell-raising in which he and the other Replacements—singer-guitarist Paul Westerberg, lead guitarist Bob Stinson, and bassist Tommy Stinson—indulged when they were together.

Also, he and Bob Stinson sabotaged a gig where he knew there would be a lot of record-industry personnel in attendance by going to a novelty store and purchasing some bottles of stink juice.

Mars sometimes transformed into an alter ego named Pappy the Clown while on tour; he painted his face when inebriated and spooked the band and road crew.

Mars appeared on only a few songs on the Replacements' final album, All Shook Down (1990); he left before the subsequent tour, unhappy with Westerberg's increasing control of the band.