Hamilton served in various campaigns in India and Africa, beginning in the 1870s, and was Lord Kitchener's chief of staff during the South African War (1899–1902). He was knighted in 1902. In 1910 he became British commander in chief in the Mediterranean.

On March 12, 1915, Hamilton was placed in charge of the expeditionary force intended to seize control of the Dardanelles Strait and to capture Constantinople. During the next six months, he conducted operations against the Turks at Gallipoli but suffered heavy casualties and made little headway. He remained unrealistically optimistic, and, when the British cabinet had begun to favour the evacuation of his force, he inopportunely reiterated his belief in the ultimate success of the campaign. He was recalled on Oct. 16, 1915, and was given no further command. He wrote Gallipoli Diary, 2 vol. (1920).