Nat Hiken cast him in the sitcom Car 54, Where Are You? as Patrolman Francis Muldoon, opposite Joe E. Ross, who played his partner, Gunther J. Toody. That program lasted two seasons, where he met longtime friend and later co-star, Al Lewis.  In The Munsters, he was given the role of Herman Munster, the tall, goofy parody of Frankenstein's monster. Gwynne was 6 ft 5 in (1.96 m) tall. For his role as Herman Munster he had to wear 40 or 50 lb (20 kg) of padding, makeup and 4-inch elevator shoes. While filming The Munsters, his face was painted a bright violet because it captured the most light on the black-and-white film. Of the face-paint, he said, "I didn't mind it all that much, although it kind of made me look like a purple Indian." Gwynne was known for his good spirits and sense of humor, and retained fond recollections of Herman.

Gwynne's last film performance was as Judge Chamberlain Haller in the 1992 film comedy, My Cousin Vinny.

The Munsters met their match when placed up against Batman which was in full color.