Mackay remained active in the Militia throughout the inter-war period. He held the rank of honorary brigadier general from 21 January 1920 to late June 1937, when he was promoted to that rank substantively. He commanded the 9th Infantry Brigade from 1 July 1920 to 30 April 1921; the 8th Infantry Brigade from 1 May 1921 to 30 April 1926; and the 5th Infantry Brigade from 1 May 1930 to 31 December 1932. On 24 March 1937 he took command of the 2nd Division. He was promoted to major general on 1 July 1937. Mackay was one of only four Militia officers to be substantively promoted to that rank between 1929 and 1939.