William Belknap was born in Newburgh, New York in 1829. He worked as a lawyer in Iowa and was a Democratic Party member of Iowa's state legislature.

Belkapp joining the Union Army on the outbreak of the American Civil War. and after fighting at Shiloh he was promoted to the rank of Lieutenant Colonel. He fought under Ulysses S. Grant at Vicksburg and was a division commander under William T. Sherman in his Atlanta Campaign. By 1865 he had reached the rank of major general.

After the war Belknap worked as a revenue collector in Iowa until being made Secretary of War by Ulysses S. Grant in 1869. Belknap became the centre of a major scandal when in 1876 he was accused of taking bribes. He was impeached and at his trial Senators voted 35 to 25 that he was guilty of corruption. However, this vote fell short of the two-thirds needed for a conviction.

Belknap practiced law in Washington until his death in 1890.