He was educated in France and at the Royal Military Academy (Woolwich) and served in the Royal Artillery during World War I. Between the World Wars, he distinguished himself in staff duties and was in charge of military training at the War Office (1936–37). Alanbrooke began service in World War II as commander of the II Army Corps in France. After the retreat to Dunkirk, he was responsible for covering the evacuation (May 26–June 4, 1940) of the British Expeditionary Force. (See World War II: The evacuation from Dunkirk (World War II).) In July he took command of the Home Forces, and he served in that capacity until he was promoted to chief of staff by Prime Minister Winston Churchill (Churchill, Sir Winston) in December 1941. He held this post until 1946. As chairman of the Chiefs of Staff Committee, Alanbrooke represented the members' views ably and firmly to the prime minister and to the U.S. Joint Chiefs of Staff and thus exercised strong influence on Allied strategy. Alanbrooke was also recognized as a brilliant field commander, though he was never given any of the great overseas commands—including, to his great frustration, command over the Allied invasion of western Europe.

After the war, published extracts from Alanbrooke's diaries provoked controversy because of their criticism of General Dwight D. Eisenhower (Eisenhower, Dwight D.)'s ability as a military commander and of U.S. strategy in general. For his military services, Alanbrooke was created Baron Alanbrooke of Brookeborough in 1945; in 1946 he became a viscount.