A railway worker at the age of 14 and later a timber merchant, he became a propagandist for Henry Mayers Hyndman's Social Democratic Federation in 1892 but eventually repudiated its strict Marxism. He helped to found (1912) and for a time edited, the Daily Herald, the first British newspaper devoted to labour subjects. In World War I he defended the rights of conscientious objectors.

A Labour member of the House of Commons (1910–12, 1922–40), he served as first commissioner of works in the Labour government of 1929–31 and then became leader of the parliamentary opposition. Unwilling to join his associates in calling for economic sanctions that might have led to war against Italy for its aggression in Ethiopia, Lansbury resigned in 1935 and was succeeded as party leader by his deputy, Clement Attlee (prime minister, 1945–51). In 1937 Lansbury visited Adolf Hitler and Benito Mussolini in the belief that his personal influence could stop the movement toward war.