Sumner was born in Carlisle Barracks, Pennsylvania on February 16, 1842. He served in the Union army during the Civil War. For much of the war, he served on the staff of his father, General Edwin V. Sumner. After the war's end, he stayed in the army and served in the cavalry during the Indian Wars.
When the Spanish-American War broke out in May 1898, Sumner was appointed brigadier general of volunteers and placed in command of the 1st Brigade, Cavalry Division, V Corps. He was sent to Cuba with the corps and led his brigade into action at the Battle of Las Guasimas. When Joseph Wheeler fell ill, Sumner temporarily assumed command of the Cavalry Division. He was still in command of the division when the fighting at the Battle of San Juan Hill began. However, it is said that when Wheeler heard the sound of battle he rose from his sick bed and rushed to take command of the division. Sumner returned to command his brigade for the rest of the battle and in the Siege of Santiago. In September, he was appointed major general of volunteers.
After the war with Spain ended, Sumner served as a military attaché to American Embassy in London. During the Boxer Rebellion, Summer was again in field command and led a brigade in the China Relief Expedition.