Donaldson joined the Australian Army on 18 June 2002. He trained at Army Recruit Training Centre, Kapooka, NSW. He won prizes as the best shot and best at physical training in his platoon. Sent to the Royal Australian Infantry Corps he then did more training at Singleton, NSW, where he again won prizes for shooting and physical training, and also as the best soldier in his platoon. He was then posted to 1st Battalion, Royal Australian Regiment, based at Townsville, Queensland.