He first acted in TV serials such as the NBC's From These Roots from 1958 to 1961 and businessman Sam Reynolds on Search for Tomorrow from 1965 to 1970. He also appeared on Broadway in the 1970 musical Applause.
His sitcom appearances prior to Soap include an auctioneer in the 1972 premiere episode of Sanford and Son, attorney Mr. Morrison in a 1973 episode of All in the Family, and Barry, gay friend of Maude on a 1974 episode of Maude. He also guest-starred in an episode of the western series Sara in 1976. He played Mr. Kirby in the 1979 NBC television adaptation of You Can't Take it With You and the ineffective but well-meaning Colonel Fielding on the 1981 TV adaptation of Private Benjamin. Roles in two movies in 1982 were Zapped! as Walter J. Coolidge and The Best Little Whorehouse in Texas as Senator Charles Wingwood.
Mandan was a frequent special guest on the CBS game show The $25,000 Pyramid during the 1980s.
In 1984–85, he played James Bradford on ABC's Three's a Crowd opposite John Ritter and in 1986 as Peace Corps member Dr. Bruce Gaines, who married Mrs. Garrett (played by Charlotte Rae) in her final episodes on The Facts of Life. He also appeared as Steven in the 1990 episode of The Golden Girls entitled "Great Expectations".
In 1991, he reunited with his former TV wife, Katherine Helmond, from Soap on Who's the Boss?. In 1993, he played Cardassian character Kotan Pa'Dar in Star Trek: Deep Space Nine episode "Cardassians", and in Married... with Children in the episode "The D'Arcy Files" (1994). His return to serials include the role of Maxwell Hammer, a friend of Minx, in Santa Barbara (1990), Mr. Jonesy alongside Louise Sorel on Days of Our Lives from 1997–98 and guest-starred as a judge on General Hospital in early 2006. Mandan has also appeared on stage throughout his career in addition to his Broadway role in Applause he starred in the pre Broadway version of the 1990s Revival of How to Succeed In Business Without Really Trying as J.B. Biggley. He also played Andrew Wyke in several touring company use of the Anthony Shaffer mystery Sleuth.
