She appeared as a guest in episodes of numerous American television shows of the 1970s and 1980s, including Emergency!, The Streets of San Francisco, Fernwood 2 Night, WKRP in Cincinnati, Barney Miller, The Jeffersons and Three's Company.

She also starred in numerous films and made-for-TV movies including Nightmare in Blood (1978), Vampire (1979), It Came Upon the Midnight Clear (1984), The Lady from Yesterday (1985) and The Scarlett O'Hara War (1980), in which she portrayed Joan Crawford.

Youngfellow is perhaps best known for her role as sharp-tongued and sarcastic waitress Jan Hoffmeyer Gray on the sitcom It's a Living from 1980–82 on ABC and 1985–89 in first-run syndication.

She and Gail Edwards, Paul Kreppel and Marian Mercer were the only actors to remain with It's a Living through the show's network and syndication runs.

In 1990, It's a Living producers Paul Junger Witt and Tony Thomas cast Youngfellow in the pilot episode of Blossom, in which she played the mother of Mayim Bialik's title character.