He attended Knox College in Galesburg, Illinois, but took a leave of absence to fight in the American Civil War for the Union Army.

[1] Cooper enlisted as a sergeant in the 137th Illinois Volunteer Infantry, and was stationed in Memphis, Tennessee, during the Confederate raid on the city by troopers under the command of General Nathan Bedford Forrest.

Upon graduation from Knox College in 1867, he returned to his hometown of Greenville and was admitted to practice law in Illinois.

In 1888, Cooper was nominated for Governor, and went on to defeat Rocky Mountain News editor Thomas M. Patterson in the general election.

Following his inauguration as the state's sixth Governor in January, 1889, he signed legislation that created thirteen new counties, including: Baca, Cheyenne, Kiowa, Kit Carson, Lincoln, Montezuma, Morgan, Otero, Phillips, Prowers, Rio Blanco, Sedgwick, and Yuma.