Mars served his acting apprenticeship in a variety of stock roles and national touring companies before appearing on Broadway in the mid-1960s, most notably in "Any Wednesday" with Sandy Dennis. He quickly moved to the small screen landing a regular role on the highly rated but short-lived Richard Benjamin-Paula Prentiss sitcom "He and She" (1967-68). In-between movie roles, Mars showed up on a number of variety and comedy specials including a stint as a regular on the short-lived "Carol Burnett and Company" (ABC, 1979), several pilots that went unsold, the occasional TV-movie, and numerous animated series, such as "Flintstone Family Adventures" (1980-81), "The New Jetsons" (1985-86), and Disney's "Darkwing Duck" and "The Little Mermaid" (1992-95).
Since "The Producers" (1967), Mars has provided outrageous performances in such comedies as Peter Bogdanovich's "What's Up, Doc?" as a German-accented swell; Mel Brooks' gothic comedy, "Young Frankenstein" (1974), as a heavily accented Transylvanian police officer, and as the slap-happy rabbi in Woody Allen's "Radio Days" (1987). Mars played the marshal in "Butch Cassidy and the Sundance Kid" (1969) and Bogdanovich cast Mars again in his less successful romantic comedy "Illegally Yours" (1988).
Mars has also had many opportunities to show a more serious side. He got favorable notices as Shirley MacLaine's husband in Frank Gilroy's dark vision of urban life, "Desperate Characters" (1971), played a former FBI agent in Alan J. Pakula's political thriller "The Parallax View" (1974), and was Lyndon B. Johnson in a docudrama about John F. Kennedy, "Prince Jack" (1984). Mars provided the voice of Triton for Disney's animated classic, "The Little Mermaid" (1989), and played a magician who takes Woody Allen on as an apprentice in "Shadows and Fog" (1992).