After a stint in the US Air Force during World War II, Fell received his degree in drama from Temple University in his native Philadelphia. He headed to New York to further his studies with famed acting teacher Stella Adler. During the early years of his career, he was often cast in smaller roles and 'pepper-up-the-scene' parts in comedies and decidedly supporting parts in dramas. Fell appeared in numerous live TV dramas in the early 1950s and made an impression on Broadway alongside Edward G. Robinson and Gene Rowlands in "Middle of the Night". He also landed his first regular TV role, as Mike, cynical best friend and fellow cab driver of Joe (Larry Blyden), on "Joe and Mabel" (CBS, 1956).

Heading to Hollywood, Fell landed his first feature role in Lewis Milestone's "Pork Chop Hill" (1959) and went on to appear in over 35 features. In 1960, he appeared with Frank Sinatra in "Oceans Eleven" and was briefly seen as the radio announcer in "Inherit the Wind". Fell appeared briefly in Mike Nichols' "The Graduate" (1967) and reteamed with the director for "Catch-22" (1970). The comic "If It's Tuesday, This Must Be Belgium" (1969) offered Fell a strong role as a husband separated from his wife (Reva Rose) during a whirlwind European tour. He also scored as the doctor more preoccupied with his own cold than with patient Burt Reynolds' condition in "The End" (1978). Fell also supported Reynolds in "Paternity" (1981). Most of his subsequent features have been unsuccessful genre fare along the lines of "Transylvania 6-5000" (1988) and "Hexed" (1993), although the Bette Midler vehicle, "For the Boys" (1991), gave Fell a good opportunity in the role of Eddie, the producer.

Television, however, has provided the best opportunities for the character actor. Fell has been a regular on several series including "87th Precinct" (NBC, 1961-62), in which he was the experienced police detective often used for comic relief. He was back as another cop in the short-lived "Dan August" (ABC, 1970), starring Burt Reynolds. Fell headlined his first comedy series in 1973, the short-lived "Needles and Pins" (NBC), set in NYC's garment district. He earned an Emmy nomination as Nick Nolte's fight trainer in the miniseries "Rich Man, Poor Man" (ABC, 1976-77). TV stardom finally arrived with "Three's Company". Fell traded ripostes with the young tenants (John Ritter, Suzanne Sommers and Joyce DeWitt) and masterly played off Lindley as his long-suffering wife. Fell's sullen, yet lovable, appearance won him fan recognition. During this period, he had a rare dramatic role as Alex Haley's agent in "Roots: The Next Generations" (ABC, 1979). Opting away from steady series work after the demise of "The Ropers", Fell was frequently seen in supporting roles in TV-movies. He made a return to series work in 1989 as one of "The Boys" (Showtime), a short-lived, pioneering pay-TV series. Additionally, Fell has appeared in TV commercials and occasional guest spots, displaying his mastery of the slow, almost constipated, slow burn.