 One of a famous family of Civil War officers, the "Fighting McCooks," he was born in Carrollton, Ohio, a son of Daniel McCook.He was educated at the United States Naval Academy at Annapolis, Maryland, and was a member of the Naval Lodge #69 of the Freemasons in New York City.When the Civil War erupted, McCook recruited a company and joined the 31st Illinois Infantry, serving under his friend, Col. John A. Logan.