Churchill was born on September 3, 1938, in London, England, the daughter and only child of Robert Churchill and his wife. Churchill's father was a political cartoonist; her mother worked as a model, secretary, and actress. Churchill began writing stories and doing shows for her parents as a child. After spending her early childhood in London, the family moved to Montreal, Quebec, Canada, in about 1949, where Churchill spent most of her formative years.

In 1956, Churchill returned to England to enter Oxford University. While studying literature at Lady Margaret Hall, she began writing plays for student productions. Her first play was written as a favor for a friend. One of Churchill's student plays, Down-stairs, won first prize at the National Student Drama Festival. Churchill graduated with her B.A. in 1960, intending to become a serious writer.

Family matters stymied her plans. In 1961, Churchill married David Harter, a lawyer, and...