A graduate of the Royal Military College at Sandhurst, Haig fought in the Sudan (1898) and in the South African War (1899–1902) and held administrative posts in India. While assigned to the War Office as director of military training (1906–09), he helped the war minister, Richard Burdon Haldane, establish a general staff, form the Territorial Army as a useful reserve, and organize an expeditionary force for a future war on the European mainland.

On the outbreak of World War I in August 1914, Haig led I Corps of the British Expeditionary Force (BEF) to northern France, and, early in 1915, he became commander of the 1st Army. On December 17 of that year, he succeeded Sir John French (afterward 1st Earl of Ypres) as commander in chief of the BEF. In July–November 1916, he committed great masses of troops to an unsuccessful offensive on the Somme River, which cost 420,000 British casualties. The next year, when the French decided to stand on the defensive until forces from the United States (which had entered the war April 6) could arrive in quantity, Haig resolved to try to defeat the Germans by a purely British offensive in French and Belgian Flanders. In the resulting Third Battle of Ypres (July–November 1917), also called the Passchendaele Campaign, the number of casualties shocked the British public, as the Somme death toll had done. But, although he failed to reach his objective—the Belgian coast—he did weaken the Germans and helped prepare the way for their defeat in 1918.

Promoted to field marshal late in 1916, Haig was steadfastly supported by King George V, but not by David Lloyd George, prime minister from December of that year. From that month to May 1917, Haig was an unwilling subordinate of the French general Robert Nivelle, supreme Allied commander on the western front. In March 1918, Haig secured the appointment of another French general, Ferdinand Foch, as Allied generalissimo. The two men worked well together, and Haig exercised full tactical command of the British armies, which had not been the case under Nivelle. After helping to stop the last German offensive of the war (March–July 1918), Haig showed perhaps his best generalship in leading the victorious Allied assault beginning August 8.

After the war, Haig organized the British Legion and traveled throughout the British Empire collecting money for needy former servicemen. He was created an earl in 1919.