William G. Donnan (June 30, 1834 – December 4, 1908) was an American lawyer, Civil War officer, politician who served as a two-term Republican U.S. Representative from Iowa's 3rd congressional district.

Born in West Charlton, a hamlet in Saratoga County, New York, Donnan attended the district schools and Cambridge Academy.

He was largely instrumental in securing the establishment of the Mental Health Institute (formerly called the Iowa State Hospital for the Insane) at Independence.

[1] In 1870, incumbent Republican Third District Congressman William B. Allison focused on winning election to the U.S. Senate, and thus declined to seek re-election to his House seat.

This article incorporates public domain material from the Biographical Directory of the United States Congress