After the war, MacMillan, having gained a Regular commission in 1915, remained in the army, continuing to serve as his battalion's adjutant until December 1920, when the battalion was stationed in Ireland during "the troubles". He was promoted to captain on 28 August 1924, serving periodically as a company commander before entering the Staff College, Camberley from 1928 to 1929, where among his fellow students there in his year included several future high-ranking officers, such as Alexander Galloway, Gerard Bucknall, John Harding, Richard McCreery, Philip Gregson-Ellis, James Hare, William Holmes, Claude Nicholson, Charles Miller, Charles Murison, Alexander Cameron, Gerald Templer, Thomas Wilson, I. S. O. Playfair and the Australian Leslie Beavis. His instructors included Henry Pownall, Wilfrid Lindsell, Richard O'Connor, Harold Franklyn, Bernard Paget, George Giffard and Bernard Montgomery. On 10 August 1929, MacMillan married Marian Blakiston Houston; they had three children. He went on to serve successively as captain, staff captain and General Staff Officer Grade 3 (GSO3) in the War Office in the early 1930s.