Youngfellow began her career in the early 1970s in a small role in a 1973 episode of The New Temperatures Rising Show. She went on to appear as a guest in episodes of numerous American TV shows of the 1970s and 80s, including The Streets of San Francisco, WKRP in Cincinnati, Barney Miller, and Three’s Company. She also starred in numerous films and made-for-TV movies including Nightmare in Blood, Vampire, It Came Upon the Midnight Clear and Moviola: The Scarlett O’Hara War in which she portrayed Joan Crawford.

She is the wife of stage and screen actor Sam Freed, whom she married in 1983. The two narrated the 2001 audiobook of The Children’s Book of Faith by William J. Bennett. Youngfellow was formerly married to Michael Mund Youngfellow from 1968-75.

Youngfellow is 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 from It’s a Living to last all the way 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; she did not continue with the project when NBC picked it up as a regular series. In 1998, she made her last TV appearance, in an episode of Law & Order.

As of 2011, she is president of the California corporation To Be Announced, Inc., founded November 20, 1978.