Sternhagen started her career teaching acting, singing, and dancing to school children at Milton Academy in Massachusetts, and she first performed in 1948 at a Bryn Mawr summer theater in "The Glass Menagerie" and "Angel Street". She went on to work at Washington's Arena Stage from 1953–54, then made her Broadway debut in 1955 as Miss T. Muse in "The Skin of Our Teeth". The same year, she had her off-Broadway debut in "Thieves' Carnival" and her TV debut in "The Great Bank Robbery" on "Omnibus" (CBS). By the following year, she had won an off-Broadway Obie Award for "Distinguished Performance (Actress)" in "The Admirable Bashville" (1955–56).