John Hay entered national service as one of Abraham Lincoln's private secretaries during the Civil War. Working with John G. Nicolay, he managed correspondence and gained close access to the president's daily business. After Lincoln's assassination he held diplomatic posts in Paris, Vienna, and Madrid, then turned increasingly to journalism and literature. His poems and prose brought him a public reputation beyond government service.

With Nicolay, Hay produced the ten-volume Abraham Lincoln: A History, published in 1890. Drawing on their access to the administration, the authors created an influential account sympathetic to Lincoln and the Union cause. Hay also served as assistant secretary of state from 1879 to 1881. President William McKinley appointed him ambassador to Britain in 1897 and secretary of state in 1898, as the United States emerged from war with Spain with new overseas possessions.

Hay's Open Door notes of 1899 and 1900 sought equal commercial access to China and preservation of its territorial and administrative integrity. The policy defended American trading interests against exclusive spheres of influence claimed by other powers; it did not give China control over the foreign privileges imposed upon it. The Boxer crisis and the international intervention of 1900 tested these declarations amid expanding imperial competition.

He remained secretary under Theodore Roosevelt after McKinley's assassination in 1901. The Hay-Pauncefote Treaty removed British treaty obstacles to an American-controlled canal, and Hay later negotiated the agreement granting the United States canal rights in Panama after its separation from Colombia. He also worked on the Alaska boundary settlement and other disputes with Britain. Roosevelt directed policy more closely, while Hay supplied diplomatic experience. He continued working despite declining health and died in office on July 1, 1905. His career joined Lincoln scholarship with diplomacy during American expansion.