Brother to Elihu Washburn, a friend to both Abraham Lincoln and Ulysses S. Grant. Washburn joined the Union army in January 1862, and elected colonel of the Second Wisconsin Cavalry. By July 1862, he had been appointed brigadier general. By November 1862, he was promoted to command the Department of West Tennessee, as a major general. He directed a detachment of three divisions of XVI Corps during siege and capture of Vicksburg. He finished the war in command of the Department of the Gulf.