Born in Evanston, Illinois, Johnston graduated from the Goodman Theatre School and later performed throughout the mid-Pacific during World War II. Sher toured in such shows as Milk and Honey, Lend Me a Tenor, and A Funny Thing Happened on the Way to the Forum. In Follies (1971), she sang the beautiful Viennese-style waltz "One More Kiss" with Victoria Mallory, who played Young Heidi. Johnston went on to appear in such Broadway shows as Sondheim: A Musical Tribute, Irene, Molly, Angel, and Me and My Girl.
Her film credits include Arthur, 9 1/2 Weeks, Fatal Attraction, Running on Empty, Eye for an Eye, and The New Guy. A member of the governing body of Actors' Equity for nearly 40 years, she also appeared frequently on television, including guest spots on Seinfeld and Mr. Rhodes.