Horner, an Iowa native, entered the United States Air Force through the Reserve Officer Training Corps program. He was commissioned in the Air Force Reserve on June 13, 1958, just before his graduation from the University of Iowa and was awarded pilot wings in November 1959 and was resworn with a regular Air Force commission in 1962. He has commanded a tactical training wing, a fighter wing, two air divisions and a numbered Air Force. While Commander of 9th Air Force, he also commanded United States Central Command Air Forces, in command of all United States and allied air assets during operations Desert Shield and Desert Storm.