Born Françoise Quoirez, she grew up in a French, Catholic, bourgeois family. She was an independent thinker and avid reader as a young girl, and upon failing her examinations for continuing at the Sorbonne, she became a writer.
She went to her family's home in the south of France and wrote her first novel, Bonjour Tristesse, at age 18. She submitted it to Editions Juillard in January 1954 and it was published that March. Later that year, She won the Prix des Critiques for Bonjour Tristesse.
She chose "Sagan" as her pen name because she liked the sound of it and also liked the reference to the Prince and Princesse de Sagan, 19th century Parisians, who are said to be the basis of some of Marcel Proust's characters.
She was known for her love of drinking, gambling, and fast driving. Her habit of driving fast was moderated after a serious car accident in 1957 involving her Aston Martin while she was living in Milly, France.
She was twice married and divorced, and subsequently maintained several long-term lesbian relationships. First married in 1958 to Guy Schoeller, a publisher, they divorced in 1960, and she was then married to Robert James Westhoff, an American ceramicist and sculptor, from 1962 to 63. She had one son, Denis, from her second marriage.
She won the Prix de Monaco in 1984 in recognition of all of her work.