Born in Bathurst, New South Wales, Mackay studied law and education at the University of Sydney.
Encouraged by Labour MP Stafford Cripps, Mackay moved to England in 1934, and began practising law.
In 1939, he took a post in the Ministry of Aircraft Production, where he became angered at Labour's reluctance to criticise government policy, and resigned from the party.
Calling for an end to the UK National Government, claiming that this would result in the more effective prosecution of World War II, his campaign gained the support of the local Constituency Labour Party, but proved unsuccessful.
Mackay later supported Tom Driberg's successful campaign in the 1942 Maldon by-election, and in 1943, he joined the Common Wealth Party, which had been formed by the merger of the 1941 Committee with Forward March.
The London Region complained that he was a careerist and an anti-Marxist, while some Christians in the organisation argued that he was destroying the party's idealism in pursuit of electoralism.