British Labour Party politician who was a noteworthy advocate of British resistance to the aggression of Nazi Germany just before World War II.

A teacher of economics, Greenwood became a civil servant during World War I and entered the House of Commons in 1922. In the first Labour Party government (January–November 1924), he was secretary to the minister of health, and in the next Labour government (1929–31) he sat in the cabinet as minister of health. From 1935 to 1945 he was deputy leader of the Labour Party. His insistence that Great Britain intervene without hesitation in the impending war influenced Prime Minister Neville Chamberlain's (Chamberlain, Neville) subsequent reversal of the appeasement policy toward Germany.

When Winston Churchill formed his coalition government in 1940, Greenwood became minister without portfolio and a member of the war cabinet and was charged with the study of reconstruction and postwar planning. He proved to be ineffective in this role, however, and was dropped in 1942. In Clement Attlee's postwar Labour government, he was lord privy seal (1945–47) and paymaster general (1946–47).