Born into the upper class, at age 15 she enrolled at the Royal Academy of Dramatic Art. Miles debuted onscreen in Term of Trial (1962), playing a student who seduces schoolmaster Laurence Olivier. She continued playing seductive roles, developing into one of the most interesting young leading ladies of the British screen in the '60s and '70s. For her work in Ryan's Daughter (1970) she received a Best Actress Oscar nomination. She was married to playwright/screenwriter/director Robert Bolt. The director Christopher Miles is her brother.