Millicent Garrett was the seventh of the 10 children of Newson Garrett, a shipowner and political radical, who for years supported the efforts of his eldest daughter, the pioneer woman physician and medical educator Elizabeth Garrett Anderson (Anderson, Elizabeth Garrett), to be admitted to the practice of medicine. In April 1867 Millicent married Henry Fawcett, a radical politician and professor of political economy at Cambridge. She helped him to overcome the handicap of his blindness, while he supported her work for women's rights, beginning with her first speech on the subject of woman suffrage (1868).

Fawcett became president of the National Union of Women's Suffrage Societies in 1897. Finally, in 1918, the Representation of the People Act, which enfranchised about 6,000,000 women, was passed. (Ten years afterward, British women received the vote on a basis of full equality with men.) In 1919 she retired from active leadership of the suffrage union, which had been renamed the National Union for Equal Citizenship.

In July 1901, during the South African War, she was sent by the government to investigate the British concentration camps for Boer civilians. Her report vindicated (whitewashed, in the opinion of some) the administration of the camps. Throughout World War I she dedicated her organization to “sustaining the vital forces of the nation.” After the war she was made a Dame of the British Empire.

Fawcett's writings include Political Economy for Beginners (1870; 9th ed., 1904), a text still in use at her death; Janet Doncaster (1875), a novel; The Women's Victory—and After (1920); and What I Remember (1924).