McCallum commenced working as a solicitor in 1986 at Mallesons Stephen Jaques (now King & Wood Mallesons) in commercial litigation, before becoming a prosecutor in the Director of Public Prosecutions, initially for the Commonwealth and then Queensland until she became a barrister in 1991. McCallum practised in a wide range of areas that included defamation, administrative law, she was counsel assisting HIH Royal Commission, represented asbestos victims in the James Hardie Inquiry,[4] and worked pro bono for refugees who were in immigration detention and in environmental matters.[2]
McCallum was appointed a judge of the NSW Supreme Court on 30 January 2008 in the Common Law Division. From 2014 McCallum was the list judge for the Defamation list,[5] and has been the trial judge in numerous high-profile defamation and criminal cases, including a long running case brought by Helen Liu in which she sought to have journalists reveal their sources,[6] and the trial of Simon Gittany for murder.[7] McCallum was the first judge in Australia to consider whether Twitter was a separate publication of defamatory material.
In considering a sexual harassment claim brought by Brigette Styles against Clayton Utz, McCallum referred to emails by another solicitor as "no advertisement for male sensitivity; their author evidently no feminist. In one of the emails, Mr Izzo speaks of 'crazy single female chicks' who 'just need a good **** to get them back to normal'. It is difficult to decide whether it is more surprising that the remarks were made at all (after over a century of feminism) or that a lawyer recorded them in an email (after over seven centuries of subpoenas).[9]
In January 2019, McCallum was elevated to the New South Wales Court of Appeal.[10]