Cook enlisted in the Union Army at age 14[1] in Cincinnati, Ohio, and served as a bugler in Battery B of the 4th U.S. Artillery Regiment.[2] During the Battle of Antietam on September 17, 1862,[2] his unit supported General John Gibbon's attack down the Hagerstown Turnpike.[1] Immediately after unlimbering their guns, the battery came under fire from Confederate infantrymen in the West Woods. Cook helped a wounded officer to the rear and, upon returning to his unit, found that most of the cannoneers had been killed. Seeing a dead artilleryman with a full pouch of ammunition, Cook took the pouch and began servicing the cannons. He continued to work as a cannoneer throughout the attack, despite intense fire from Confederate soldiers who came within 15 feet of the guns.[1]
The next year, Cook participated in the Battle of Gettysburg, where he carried messages across a half-mile of fire-swept terrain. During that battle, he helped destroy a damaged caisson to prevent it from falling into the hands of approaching Confederates.[1]
For his actions at Antietam, Cook was awarded the Medal of Honor several decades later, on June 30, 1894.[2] His official Medal of Honor citation reads:
Volunteered at the age of 15 years to act as a cannoneer, and as such volunteer served a gun under a terrific fire of the enemy.[2]