 At the outbreak of war in 1939, Mackay was ranked seventh on the army's seniority list.Following formation of a second infantry division for the Second Australian Imperial Force in 1940, Lieutenant General Sir Thomas Blamey was elevated to command of the newly created I Corps.Mackay was selected to command the 7th Division on the advice of General Sir Brudenell White but Cabinet, after consulting with Blamey, switched this appointment to the 6th Division.