 McDonald worked as a paediatrician at Campbelltown Hospital for 17 years before his election to Parliament.In this period, he was quoted by the "Sydney Morning Herald" as a critic of Opposition Leader John Brogden's policies on health.On 23 January 2007, he was preselected for the New South Wales electoral district of Macquarie Fields by the Labor Party, following the disendorsement of incumbent MP Steven Chaytor.