 From 1913 to 1914, he studied for a Diploma of Military Science course at the University of Sydney.Mackay joined the Australian Imperial Force on 27 August 1914 as adjutant of the 4th Infantry Battalion, with the rank of captain.On 4 September 1914, he married his fiancée, Marjorie Eveline Meredith, the daughter of Lieutenant Colonel John Meredith, in a ceremony at St Philip's Church, Sydney.