Attlee entered Parliament in 1922. In 1927 he visited India as a member of the Simon commission and was converted to views that strongly favored Indian self-government. He joined the Labour government in 1930 but resigned in 1931 when Ramsay MacDonald formed the National government. As leader of the Labour party from 1935, Attlee was an outspoken critic of Conservative foreign policy, objecting particularly to the government's failure to intervene in the Spanish civil war. During World War II he served (1940-45) in Winston Churchill's coalition cabinet, and on Labour's electoral victory in 1945 he became prime minister.

Under Attlee's leadership, the Bank of England, the gas, electricity, coal, and iron and steel industries, and the railways were nationalized. His government also enacted considerable social reforms, including the National Health Service. Independence was granted to Burma (Myanmar), India, Pakistan, Ceylon (Sri Lanka), and Palestine, and Britain allied itself closely with the United States in the cold war confrontation with the Soviet Union. The postwar economic crisis required stringent economic and financial controls, which reduced support for the government. Labour won the 1950 general election by a narrow margin, but in 1951, Attlee decided to go to the country again and was defeated. He was leader of the opposition until his retirement in 1955, when he received the title of Earl Attlee.