 According to Don Whitington, McMahon's life before entering politics was "the aimless, indolent existence of a wealthy young man with a position in a big city's smart set, no positive ambition or even interests, except in enjoying himself, and no family ties to give him a feeling of responsibility or even consideration for others".After graduating from university, he secured a position as a solicitor with Allen, Allen & Hemsley, a major Sydney law firm; he was made a junior partner in 1939.He was assigned to the Commonwealth Bank and the Bank of New South Wales for periods, which helped spark his interest in economics.