De Munn made his motion picture debut opposite Ellen Burstyn in "Resurrection" (1980). He went on to play numerous supporting roles, including Harry Houdini in Milos Forman's "Ragtime" (1981), a moodily and sullen Clifford Odets opposite Jessica Lange in "Frances" (1982), a provincial sheriff who doesn't know what to make of "The Blob" (1988) and the father of one of the leaders of the newsboy strike in "Newsies" (1992). In 1994, De Munn was the D.A. who helps to convict Tim Robbins of murder in the early sequences of Frank Darabont's "The Shawshank Redemption" and co-starred in Jon Turtletaub's "Phenomenon" (1996), as a professor investigating John Travolta's newfound abilities.

On the small screen, De Munn has become a frequent presence in high profile made-for-cable films and PBS specials. He made his TV debut in "The Last Tenant" (ABC, 1978), played Doc Holliday in "I Married Wyatt Earp" (NBC, 1983) and was a doctor offered Veronica Hamel a real relationship in "Sessions" (ABC, 1983). Among his notable appearances on PBS are "Mourning Becomes Electra" and several productions for "American Playhouse" (e.g., "The Joy That Kills" 1985, "Eugene O'Neill: Journey into Genius" 1988, as George Pierce Baker). Other notable credits include playing the adult magician in "Young Harry Houdini" (ABC, 1987), a cautious paper clip-counting executive in "Barbarians at the Gate" (HBO, 1993), Robert Oppenheimer in "Hiroshima" (Showtime, 1995), a Russian serial killer in "Citizen X" (HBO, 1995), for which he received a CableACE Award, and Mel Korn, one of the anchorwoman's husbands in "Almost Golden: The Jessica Savitch Story" (Lifetime, 1995).