Born May 16, 1840, Sumner was one of eight siblings who grew up on a farm in Muscatine, Iowa. He became an enthusiastic reader but was not interested in farming. During the American Civil War, he became a corporal and sharpshooter in the 32nd Iowa Volunteer Infantry, fighting for the Union at Vicksburg and Nashville. When fighting in the Battle of Pleasant Hill in 1864, both of Sumner's legs were broken and both of his hips were dislocated by an exploding shell, and a shell fragment hit his head. Although he recovered, he was left with recurrent headaches.