In 1956 Lady Antonia married Hugh Fraser, MP with whom she had six children. A Roman Catholic, she caused a public scandal in 1977 by leaving her husband to live with the playwright, Harold Pinter, whom she eventually married, in 1980. Pinter's then-wife, the actress Vivien Merchant, spoke publicly of her distress at his abandonment of her and made cutting remarks about Fraser in the press, including the famous comment that "she has very big feet".
Fraser was educated at Lady Margaret Hall, Oxford. Her first major work was Mary, Queen of Scots (1969). She followed it up with various other biographies, including, Cromwell, our Chief of Men (1973), and she won the Wolfson History Award in 1984 for The Weaker Vessel, a study of women's lives in 17th century England. She also writes detective novels.