She studied at the Royal Academy of Dramatic Art in London. In 1960 she made her first movie, Tunes of Glory, costarring with Alec Guinness and John Mills. She appeared in the notable films A Man for All Seasons (1966), The Killing of Sister George (1968) and Battle of Britain (1969). She was nominated for a Best Supporting Actress Oscar for They Shoot Horses, Don't They? (1969). In 1972 she won the Best Actress award at the Cannes Film Festival for her role in Images.

Her writing career is less well-known. In the 1970s she wrote two children's fantasy novels, In Search of Unicorns (1973), revised (1984), allegedly filmed as Images, and Lark's Castle (1976), revised (1986).

In 1960 she married Michael Wells. The couple had two children, Orlando and Sasha, but they divorced in 1980. In the 1984 TV adaptation of A Christmas Carol, she played Mrs. Cratchit and both of her children co-starred as Cratchit offspring. Her son, Orlando Wells, currently stars in the Channel 4 teen drama, As If.

Politically she is a leftist who has publicly supported Mordechai Vanunu, the Israeli dissident who revealed Israel's nuclear weapons programme.