Kammala jumla mai zuwa: A lokacin Yaƙin Duniya na Biyu, MacMillan ya fara aiki a Ingila, yana tsara dabarun kare kai game da yiwuwar mamayewar Jamusawa. An nada shi Birgediya Janar Janar na IX Corps a watan Disamba na 1941, ya kasance a wannan mukamin yayin saukar da Torch a Arewacin Afirka har zuwa faduwar Tunis a watan Mayu na 1943. An ba shi umurnin Brigade na 152 a watan Yunin 1943 kuma ya jagoranci shi a lokacin nasarar da aka samu
Yaƙin neman zaɓe na Sicily. Bayan ya dawo Biritaniya, an ba shi umurnin rundunar ta 15 (Scottish) kuma ya jagoranci tsari a lokacin yakin Normandy, Operation Epsom da Operation Bluecoat, zuwa karshen abin da ya ji rauni. Da zarar ya dawo, a watan Nuwamba 1944, ya koma babban yankin Turai a matsayin GOC 49th (West Riding) Division kusa da Nijmegen. Bayan mutuwar Manjo-Janar Thomas Rennie, ya karbi umurnin 51st (Highland) Division nan da nan bayan tsallaka Rhine a ranar 23 ga Maris, 1945.