Robert Mitchell was born on April 4, 1823, in Mansfield, Ohio, and served in the Mexican-American War; in 1856 he moved to Linn County, Kansas Territory, and became active in territorial politics. From 1859 to 1861 he served as treasurer of the Kansas Territory. Commissioned colonel of the 2nd Kansas Infantry in May 1861, Mitchell led the regiment at the Battle of Wilson’s Creek, where he was badly wounded. After recovering, he recruited the 2nd Kansas Cavalry and was appointed a brigadier general by President Lincoln on April 8, 1862.

He led a division at the Battle of Perryville, Kentucky in October 1862, and served as chief of cavalry of the Army of the Cumberland during the Chickamauga campaign; just before the Confederate siege of Chattanooga was raised, he was ordered to Washington, D. C., for court-martial duty.

Mitchell commanded districts in Nebraska and Kansas in 1864 and 1865. Mustered out on January 15, 1866, he was confirmed the same day by the United States Senate as governor of the New Mexico Territory. Resigning in 1869, he returned to Kansas, but eventually moved to Washington, D. C., where he died on January 26, 1882. Mitchell is buried in Arlington National Cemetery.