Diane Cilento was born in Mooloolaba, Queensland, Australia. Her parents, Sir Raphael Cilento and Phyllis Dorothy McGlew, Lady Cilento, were both doctors. She decided to become an actress at an early age, and after being expelled from school in Australia, was educated in New York while living there with her father. At age 17, she won a scholarship to the English Royal Academy of Dramatic Art (RADA) and moved to London in the early 1950s. After graduation, she found stage work almost immediately and then was signed to a five-year movie contract by Sir Alexander Korda. Her first leading role in films was in Passage Home (1955). In 1956, she was nominated for a Tony Award for Best Supporting Actress for her role as Helen of Troy in Jean Giraudoux's Tiger at the Gates.
She was nominated for an Oscar for Best Supporting Actress for her performance as Molly Seagrim in Tom Jones (1963). During her marriage to actor Sean Connery (1962-1973), the second of her three husbands, she took few movie parts, notably in the American western Hombre (1967).
One of her best-known part was in The Wicker Man (1973), her last British picture before returning to her homeland. In 1985, she married playwright Anthony Shaffer and they settled north of Cairns, Australia, where she built her own outdoor experimental theatre. She published her autobiography, My Nine Lives (2006).