Born into a working-class Evangelical family in Footscray, Melbourne, Australia, he moved to Britain in 1971. Already an activist, he joined the Gay Liberation Front in London. The failure to stop the anti-gay Clause 28, prohibiting the "promotion" of homosexuality, helped convince him and other activists to form OutRage! in 1990.

He outed a bishop of the Church of England. In march 2001 Tatchell tried to make a citizens arrest of the Zimbabwe President Robert Gabriel Mugabe in the Hilton Hotel in Brussels, Belgium, on charge of torturing Ray Chote and Mark Chavundula. Tatchell was beaten up and knocked unconcious by Mugabe's bodyguards.