She was born Cicily Isabel Fairfield in County Kerry, Ireland, the daughter of a soldier and war correspondent. After attending George Watson's Ladies College, Edinburgh, she appeared for a short time on the London stage, notably in Ibsen's Rosmersholm; she took her pen name from the heroine of this play. From 1911 West became involved in women's suffrage campaigns and turned to journalism; throughout her life she continued to contribute to leading British and American periodicals, beginning with the feminist Freewoman (which her mother had forbidden her to read). She joined The Clarion the following year as a political writer and later reviewed novels for The New Statesman and contributed to The Daily Telegraph.

In about 1913 Rebecca began a ten-year affair with the novelist H. G. Wells. Their son, Anthony West, who also became a writer, was born in 1914. In 1916 her first full-length book appeared, a critical study of Henry James. Her first novels, The Return of the Soldier (1918), about the effects of shell shock, and The Judge (1922), a study of the Oedipus complex, show the impact of Freudian psychology on her thinking. All her later novels, which appeared at irregular intervals, show this same psychological interest. They include Harriet Hume (1929), The Harsh Voice (1935), The Thinking Reed (1936), and, after a gap of 20 years, The Fountain Overflows (1956) and The Birds Fall Down (1966).

In 1923, after breaking with Wells, West went to the United States and began contributing to the prominent American journals The New Republic and The New Yorker. After travelling in 1937 to Yugoslavia with her husband, Henry Maxwell Andrews, a banker whom she had married in 1930, she produced Black Lamb and Grey Falcon (1941). A controversial two-volume travel diary that expands into a cultural and political examination of Balkan history, it is generally considered her greatest work. During World War II she supervised BBC broadcasts to Yugoslavia. In 1945 she was highly acclaimed for her coverage in The New Yorker of the trial of William Joyce (“Lord Haw-Haw”), who was tried for treason for broadcasting Nazi propaganda to Britain and subsequently executed. This was later published as The Meaning of Treason (1949) and expanded and updated in 1965 to include material on the communist traitors Burgess, Maclean, Philby, and Blake. Her reports on the Nuremberg trials of German war criminals were collected in A Train of Powder (1955).

Rebecca West's other nonfiction works include D. H. Lawrence (1930), St. Augustine (1933), The Modern Rake's Progress (1934), McLuhan and the Future of Literature (1969), and 1900 (1982). She was created an OBE in 1949 and DBE in 1959.