Mary Hayley Bell was born in Shanghai, China, where her father served as commissioner for maritime customs. She was educated by a governess and attended school in Hong Kong before being sent to England to attend Sherborne and Malvern Girls' College. She went to the Royal Academy of Dramatic Arts and in 1932, won a role in The Barretts of Wimpole Street, performed in Shanghai by an American touring company. Two years later, she began appearing on the London stage and in films. Her acting career ended on her marriage in 1941 to John Mills, with whom she had four children, and she transitioned to a new career as a writer. Her plays included Men in Shadow (1942), Angel (1947), Duet for Two Hands (1945), and The Uninvited Guest (1953). She also co-wrote the screenplay and story of Sky West and Crooked (1966), released as Gypsy Girl in the USA, and wrote additional dialogue for Scott of the Antarctic (1948). Her first novel Whistle Down the Wind (1948) was adapted into a highly successful 1961 film starring her daughter Hayley Mills and into a 1998 musical by Andrew Lloyd Webber. Her autobiography, What Shall We Do Tomorrow? was published in 1968.