Harmon was born in Chicago, Illinois. She attended The Second City troupe in Chicago until moving to Los Angeles to pursue an active acting career. She is probably best remembered for her role as Elizabeth Lubbock in the ABC television series Just the Ten of Us which was a spin-off of Growing Pains where she first appeared in the role for two episodes in 1988. She also was a regular on the short-lived television series The Ted Knight Show and Leo & Liz in Beverly Hills. She has also had parts in films such as Bachelor Party (1984) and Back to the Future (1985) and co-starred with Kurt Russell in the 1980 comedy Used Cars.