Robert Mactier was born at Tatura, Victoria, on 17 May 1890, the seventh of ten children of Robert and Christina Mactier of "Reitcam" ("Mactier" spelt backwards), a nearby property. In his formative years, he was a keen sportsman. He went to school locally, and later worked on his father's properties until he enlisted in the Australian Imperial Force at Seymour on 1 March 1917. He sailed from Melbourne with the other 151 Victorians of the 23rd Battalion's 19th Reinforcements aboard HMAT A11 "Ascanius " on 11 May. He arrived in France on 14 November, and joined the battalion on 23 November. Posted to B Company, he fought in all of the battalion's subsequent battles until his death, including at Hamel, the August Offensive and Albert, where he was gassed. His brother, David, served with the 37th Battalion.