Mallard was President of the New South Wales branch of the Young Liberals from 1993 to 1994, succeeding John Brogden, who later became leader of the NSW Liberal Party and NSW Leader of the Opposition. Prior to election to the City of Sydney Council in March 2004, Mallard served as a councillor on the SouthSydney City Council, from July 2000. In 2004, both Councils were amalgamated. Mallard is the first Liberal endorsed candidate elected to the City of Sydney. In 2008, Mallard ran against sitting candidate Clover Moore for the popularly elected position of Lord Mayor, gaining 14% of the vote. Moore was re-elected. Mallard was instrumental in setting up the City's register of same-sex relationships. Mallard has long been supportive of marriage equality, putting him in opposition to many party members, including the former Federal Leader of the Liberal Party and Prime Minister of Australia, Tony Abbott but putting him in support with the former Prime MinisterMalcolm Turnbull. In 2009, Mallard faced two accusations of misconduct and for breaching Council's code of conduct. The first alleged indiscretion related to debating and voting on a development application before Council from the hotelier George Thomas who had allegedly made a donation to support Mallard's re-election to Council. Using parliamentary privilege, Lee Rhiannon, a Greens Member of NSW Legislative Council, accused Mallard of a second indiscretion by receiving illegal donations through provision of rent by a third party during the 2008 local government election campaign. Mallard asked the matters be referred to the NSW Department of Local Government for investigation. The Department of Local Government investigation cleared Mallard of allegations that he breached the Council's code of conduct. Following an earlier announcement that he would again run against Moore for the popularly elected position of Lord Mayor, in June 2012 Mallard announced that he would not contest the 2012 local government elections. In September 2012, Mallard was endorsed as the Liberal Party candidate for the NSW Legislative Assembly seat of Sydney at the 2012 by-election but was beaten by Moore-backed independent candidateAlex Greenwich.
Mallard's early career was in marketing and advertising. In 1995 he took a career sabbatical and established a small business in horticulture with his brother in the inner city suburb of Darlinghurst. Mallard, who is gay, is in a same-sex relationship with Danish born Jesper Hansen. On 20 July 2012 it was reported that they would marry in a Lutheran Church in Copenhagen in 2013.