His cartoons were first published in 1983 in The Bulletin and after he drew for The Sydney Morning Herald until 1994, when he was recruited by News Limited to contribute to The Daily Telegraph-Mirror and later to The Australian.

As an artist and illustrator, Leak was acclaimed by journalist Peter FitzSimons as "colossally talented, driven, and passionate for his craft".

In April 2006, Leak drew a cartoon captioned "No Offence Intended", depicting an Indonesian person resembling then president, Susilo Bambang Yudhoyono, as a dog mounting a Papuan native.

The foreign minister, Alexander Downer, told media that he felt Leak's cartoon was crude, offensive and potentially racist.

The academic Amanda Wise, an associate professor of sociology at Macquarie University, told media that it was her view that the cartoon was racist.

[22] Western Australian Police Commissioner Karl O'Callaghan and academic Jeremy Sammut defended Leak's 2016 cartoon, saying it was an appropriate portrayal of some communities and families.

[29] On 21 September 2016, during a nationwide debate about legalising same-sex marriage (SSM), The Australian published a Leak cartoon depicting a club-carrying, goose-stepping platoon, wearing rainbow-coloured NAZI SS uniforms, captioned "Waffen-SSM", which provoked significant controversy.

[2][34] He entered portraits of Malcolm Turnbull in 1994, Graham Richardson in 1995, Tex Perkins in 1997, Gough Whitlam in 1998, Sir Les Patterson in 2000 and Robert Hughes in 2001.

[38] On 18 October 2008, Leak sustained serious head injuries from falling off a balcony while trying to feed African grey parrots and gang-gang cockatoos.

[40] His partner Lo Mong Lau, along with his elder son Johannes and his mother and sister, joined him to be by his side at the Royal North Shore Hospital where he was treated.

[52] The reviewer Gillian Dooley wrote that the book was not a success, labelling the first half "tedious, crude, self-indulgent and melodramatic" and the end "truly nauseating".

[38] Portraits painted for the show included Australian actor June Salter, musician Bon Scott and Aboriginal activist Charles Perkins.