After graduation from West Point, in 1851, Gillem served in Florida and Texas prior to the start of the war. He received several assignments, in which he served at Mill Springs, Shiloh and Corinth. He received the rank of brigadier general in 1863, in which position, he commanded the cavalry division of the Army of the Cumberland. At the end of the war, Gillem remained in volunteer service until September 1866 when he received a commission for the regular United States Army with a promotion to major general.
