 Upon his return to Ohio in 1859, he studied law in the family business, Stanton & McCook, and was admitted to the bar.However, he did not get to establish his own legal career as the Civil War erupted and McCook offered his services to the army, following President Abraham Lincoln's call for volunteers to put down the rebellion.McCook enlisted in the 2nd Ohio Infantry, a three-months regiment, and was commissioned as a captain.