The younger King entered the army as a private in Company F, 7th New York Militia, serving a three-month enlistment from April to June 1861. He acquired a direct commission as a first lieutenant in the regular U.S. Army, and was assigned to the 4th U.S. Artillery, on August 5, 1861. He served in the Army of the Potomac throughout the war, eventually commanding (from 1864) 4th U.S. Light Artillery, Battery A, in the famed U.S. Horse Artillery Brigade. Lieutenant King was awarded one brevet promotion (to captain) for his bravery during the Seven Days Battles, in actions at the Battle of White Oak Swamp, on June 30, 1862. During that fight, King ranked only as a section chief, took command of combined Batteries A &amp; C, 4th U.S. Artillery, when his commander was wounded.