He appeared in two Mel Brooks films: as the deranged Nazi playwright Franz Liebkind in The Producers (1967) and Police Inspector Hans Wilhelm Friedrich Kemp in Young Frankenstein (1974).

Mars appeared in two seasons of Malcolm in the Middle as Otto Mannkusser, Francis's well-meaning but dimwitted boss and a German immigrant who owns a dude ranch.

He also did several other animated voice-over film roles such as Littlefoot's grandfather in the Land Before Time series (1994–2008) and that of Professor Screweyes in We're Back!

He later appeared on such television series as Gunsmoke, Get Smart, McMillan & Wife, Columbo, Harry O, The Bob Crane Show, and Police Woman.

He was cast opposite Bette Davis in Hello Mother, Goodbye!, a 1973 television pilot aired by NBC but never picked up as a series.

[4] In 1969, Mars portrayed a marshal trying to raise a posse to pursue Butch Cassidy and the Sundance Kid, only to have his address to the townsfolk hijacked by a friendly bicycle salesman.

He portrayed German characters in The Producers (1967) and Young Frankenstein (1974), and played a Croatian musicologist, Hugh Simon, in What's Up, Doc?

His first broadly accented character was that of Sir Evelyn Oakleigh in the 1962 Off-Broadway revival of the Cole Porter musical Anything Goes.

He voiced characters on many animated television series, such as The Smurfs, The Biskitts, A Pup Named Scooby-Doo, TaleSpin, and Animaniacs, as well as video games such as Fallout and Kingdom Hearts.