He was a visiting fellow with the Department of History at the Australian Defence Force Academy from 1985 to 1988, and a member of the directing staff at the Joint Services Staff College in 1988 to 1990. Horner retired from the full-time army in 1991 on gaining a position with the Australian National University (ANU) and transferred to the Australian Army Reserve, with which he served for more than a decade. He was the inaugural commanding officer of the Land Warfare Studies Centre (1998–2002), and retired with the rank of colonel.
He held a visiting fellowship at the Department of History at the Australian Defence Force Academy from 1985 to 1988, and served as part of the directing staff at the Joint Services Staff College in 1988 to 1990. Horner left active duty in 1991 when he took up a position with the Australian National University (ANU) and joined the reserve forces, where he remained for over a decade. He was the first commander of the Land Warfare Studies Centre (1998–2002), and retired as a colonel.