Their grandfather George McCook had participated in the Irish Rebellion of 1798 and escaped to the United States after it was defeated.

[1] At the start of the Civil War, McCook received a commission as colonel of the 1st Ohio Infantry in April 1861.

On September 3, 1861, he was promoted to brigadier general of volunteers, commanded a brigade in Kentucky that fall, and led a division by February 1862.

He earned the brevet of lieutenant colonel in the regular army for his part in the capture of Nashville, Tennessee.

McCook then commanded the 2nd Division in the Army of the Ohio at the Battle of Shiloh on the second day of fighting, and then in the subsequent campaign against Corinth.

McCook's demotion from command was in large part political, as he was a pro-slavery Democrat and had earned the ire of Secretary of War Edwin Stanton.

McCook resigned from the volunteer service in October 1865 and reverted to the regular army rank of captain.

One of his final actions before retirement was helping send Army troops to break up the 1894 Chicago railroad strike.