Towards the end of his senior year at Annapolis, the American Civil War erupted and Kempff was detached from the Naval Academy and called into active service in May 1861. Midshipman Kempff was assigned to the , a steam frigate in the Atlantic Blockading Squadron. On November 7, 1861, Kempff participated in the Battle of Port Royal, near Hilton Head Island, South Carolina. On August 1, 1862, Kempff was commissioned as a Lieutenant and subsequently assigned to the . At the close of the Civil War, Kempff was aboard the in the Pacific Ocean.