Susan Mary Cooper was born on May 23, 1935 in Burnham, Buckinghamshire, England. She attended Slough High School, subsequently earning a degree in English from Oxford University. Cooper resided in Buckinghamshire until she was twenty-one, when her parents moved to her grandmother's village of Aberdovey, Wales. Following graduation, she worked as a reporter for The Sunday Times of London under Ian Fleming.
It was during this time that she finished her first novel, Mandrake, and began her most notable work, The Dark is Rising, an award-winning five-volume fantasy saga set in and around England and Wales. Her books include traditional British mythology, interwoven with original material. She has written for both children and adults.
In 1963 she left England and emigrated to the United States to marry a professor at the Massachusetts Institute of Technology. She then began to write full-time, focusing on the Dark is Rising series and her 1970 autobiography, Dawn of Fear.
Cooper married Canadian-American actor Hume Cronyn, widower of the late Jessica Tandy, in July 1996. She currently resides in Fairfield, Connecticut.