After being educated at Haileybury College and at New College, Oxford, Curtis entered the legal profession. He fought in the South African War, became secretary to Sir Alfred Milner, British high commissioner in South Africa, whose staff of gifted young men became known as “Milner's kindergarten.” Curtis also filled several posts in the Transvaal government. In 1907 he resigned to work for the union of the four British colonies in South Africa, and he began to develop a conception of a federal world order that occupied him for the rest of his life.

In 1910 Curtis founded the quarterly Round Table for the propagation of Liberal imperialist thought, and in 1912 he was appointed Beit lecturer in colonial history at Oxford University. In 1920 Curtis helped found the organization that became, in 1926, the Royal Institute of International Affairs. From 1921 to 1924 he served as colonial office adviser on Ireland.

Curtis' first major book was The Commonwealth of Nations (1916). He was chiefly responsible for replacing the term “empire” with “commonwealth.” His visits to India and China gave him material for Dyarchy (1920) and The Capital Question of China (1932). After 1932 he devoted himself to his most important work, Civitas Dei, 3 vol. (1934–37), in which he advocated a world federation.