After primary flight training at Lackland AFB, jet training at Laredo AFB, gunnery training at Williams AFB, top-off training and F-100D transition training at Nellis AFB, Horner's first operational assignment was in October 1960 with the 48th Fighter Wing at RAF Lakenheath. In 1963, Horner was reassigned to the 4th Tactical Fighter Wing and Seymor Johnson AFB, where he flew the F-105. From April through August 1965, Horner was assigned as an F-105 pilot at Korat BA in Thailand, flying missions over North Vietnam. From August 1965 to 1967, Horner returned to Nellis AFB as an F-105 instructor, becoming involved in a number of projects involving other aircraft and undergoing F-105 Wild Weasel training. In 1967, he returned to Korat AB, flying both Wild Weasel and night radar bombing missions.