Born Annie Wood in Clapham, London, her parents were of Irish origin. At the age of nineteen in 1867 she married an Anglican clergyman, Frank Besant (younger brother of Walter Besant the novelist, historian and freemason). They had two children but separated in 1873 by which time she had become involved in radical causes. Throughout her life she was a brilliant public speaker.
By 1877 she was the deputy to atheist Charles Bradlaugh in the National Secular Society (founded 1866), and was co-defendant with him in the prosecution for publishing a new edition of The Fruits of Philosophy, a guide to birth control and sexual health by american doctor Charles Knowlton (1800-1850). They were acquitted on a technicality, and the case led to the formation of the Malthusian League, for better sex education, which lasted until 1927.
Besant and Bradlaugh supported the campaign for Irish Home Rule. She befriended the young Irish author George Bernard Shaw (1856-1950) in London, and through him became involved with the socialist Fabian Society in the 1880s. In 1887 there was a riot at a rally for unemployed people in Trafalgar Square at which she spoke. In 1888 she took a prominent role in the successful matchgirls strike at the Bryant and May factory in Bow, which significantly improved pay and ameliorated the appalling conditions and enabled union organisation.
In 1889 she met Madame Blavatsky and became a Theosophist, and when Blavatsky (and Bradlaugh) died in 1891 she became a leading figure in the movement, which however fragmented. From 1893 Mrs Besant spent much time in India, promoting theosophy and campaigning for Indian freedom and secular government. In 1917 she was briefly the head of the Indian National Congress Party, but left when Mohandas Ghandi joined from South Africa and advocated civil disobedience.
In 1898 she helped set up the central Hindu College for boys, which became the nucleus for Benares Hindu University. Among the boys she discovered and adopted Jiddu Krishnamurti (1895-1986) whom she declared to be a new Messiah, although Krishnamurti later repudiated these claims.