Mills began her career as a child actress and was nominated at age eighteen for a Tony Award for her stage performance in "Five Finger Exercise" in 1960. She progressed to film work and then to television, playing the lead role on the sitcom "Nanny and the Professor" in the early 1970s. She received Golden Globe Award nominations for her work in this series and for her role in the film "Avanti!" in 1972. She won an Emmy Award for her performance in the television miniseries "QB VII" (1974). In 1983, Mills joined The Mirror Theater Ltd's Mirror Repertory Company, performing in repertory productions such as "Rain", "Paradise Lost", "Inheritors" and "The Hasty Heart" throughout their seasons. From 1999 until 2008, she had a role on the daytime drama series "Passions", for which she was nominated for a Daytime Emmy Award.