Mars was born in Chicago. His father, Bernard "Sonny" Mars, was a radio and television personality. He studied fine arts and acting at Northwestern University.
After graduation he began acting in the early 1960s, then did small roles in theatre followed by TV and film. In 1977, he married Barbara Newborn. They had two daughters, Susannah and Rebecca, and six grandchildren. The marriage lasted until his death in 2011.
Mars made his acting debut in 1962 as a book publisher on Car 54, Where Are You?. He later appeared on such television series as Gunsmoke, Get Smart, McMillan & Wife, Columbo, and The Bob Crane Show. He also appeared in dramatic roles, such as Will Turner, a former FBI agent, in Warren Beatty's The Parallax View.
Mars played Harry Zarakartos on the Richard Benjamin-Paula Prentiss sitcom He & She. He was featured in a number of small roles in broadcasts such as the Misfits of Science pilot episode and the Star Trek: Deep Space Nine episode "Shadowplay". He was cast opposite Bette Davis in Hello Mother, Goodbye!, a 1973 television pilot aired by NBC but never picked up as a series. From 1970-74 Mars guest starred in five episodes of Love, American Style, playing various characters.
In 1977, Mars became a series regular on both the Sha Na Na variety series and on Norman Lear's talk show parody Fernwood 2-Night in the memorable recurring role of William W.D. 'Bud' Prize, appearing the following year on America 2-Night in the same role. In 2001 Mars portrayed a comedic famous but washed-up photographer on Just Shoot Me. Before his death, his final television roles were Otto, the German dude ranch owner on Fox Broadcasting Company's Malcolm in the Middle, an appearance on Disney Channel's Hannah Montana, and a reprise of his role as Grandpa Longneck in The Land Before Time television series.
Mars often played characters with exaggerated accents. He portrayed German characters in The Producers and Young Frankenstein, and played the 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 also appeared in the 1962 Broadway play, The Affair.
In 1975, ABC/Dunhill released a comedy LP produced by Earl Doud, Henry the First, featuring Mars in a number of comedy bits as Henry Kissinger, including a cover version of the Bachman–Turner Overdrive song, "Takin' Care of Business".
Mars cultivated a lengthy voice acting career, launching it by voicing several characters on Uncle Croc's Block. He voiced the roles of Ariel's father King Triton in The Little Mermaid and the villainous Professor Screweyes in We're Back! A Dinosaur's Story, as well as voicing Triton in Kingdom Hearts and Kingdom Hearts II.
He voiced Littlefoot's Grandpa Longneck in the Land Before Time series of films and the spin-off television series. He played some minor roles on the popular radio show, Adventures in Odyssey. He played Sweet William in Fievel's American Tails, which took place after An American Tail: Fievel Goes West. He voiced characters on many animated television series, such as The Smurfs, The Biskitts, A Pup Named Scooby-Doo, Tale Spin, and Animaniacs, as well as video games, such as Fallout and Kingdom Hearts.
In 2008, Mars was diagnosed with pancreatic cancer which had already spread beyond his pancreas. His cancer made him so ill that he could not reprise his role as King Triton for The Little Mermaid: Ariel's Beginning, so Jim Cummings took over the role. As well as not being able to voice Triton, he could not voice Grandpa Longneck in The Land Before Time XIII: The Wisdom of Friends (Grandpa does appear in the film, but has no lines).
Mars died from pancreatic cancer on February 12, 2011, at the age of 75.