Frankau was born in London, the younger daughter of Dorothea Frances Markham Drummond-Black and the novelist
Gilbert Frankau. Her grandmother was the satirist
Julia Frankau, one of several famous siblings, and her uncle was the British radio comedian
Ronald Frankau. Never attentive to his two daughters, her father abandoned the family for another woman in 1919, and Frankau and her elder sister Ursula were sent as boarders to
Burgess Hill Girls (previously named Burgess Hill School for Girls) in Sussex until 1924. Frankau wrote about this period in her autobiographical novel
I Find Four People (1935). She had success as a writer from a young age.
Marriage of Harlequin (1927), her first novel, was written at age 19 and well received by critics. Over the next dozen years, she published 20 novels. She had a long but stormy friendship with the author and journalist Dame
Rebecca West. A long relationship with the married poet
Humbert Wolfe ended with his death in 1940. Frankau then ceased to write for almost 10 years. During the Second World War, she worked for the BBC, the Ministry of Food and then with the
Auxiliary Territorial Service, where she began a lesbian affair with fellow officer Marjorie Vernon Whitefoord, who sponsored Frankau's conversion to Roman Catholicism in 1942. In 1945, she married Marshall Dill Jr. (1916 – 2000), a former American naval intelligence officer. After the war, Dill became a university professor. They resided in California. In 1946, their only child, Anthony Marshall Dill, died in infancy, resulting from complications due to premature birth. The couple divorced in 1951, and Frankau later returned to London ==Later years==