Horrocks has been involved in the New Zealand comic scene since the mid 1980s, when he co-founded Razor with Cornelius Stone and had his work published in the University of Auckland student magazine Craccum. Later in the decade he began to get international recognition, having work published by Australia's Fox Comics and the American Fantagraphics Books. He then moved to the United Kingdom where he self-published several mini-comics and co-founded Le Roquet, a comics annual. Upon returning to New Zealand in the mid 1990s, Horrocks had a half-page strip called 'Milo's Week' in the current affairs magazine New Zealand Listener from 1995 to 1997. He also produced Pickle, published by Black Eye Comics, in which the 'Hicksville' story originally appeared. Hicksville was published in book form in 1998, achieving considerable critical success. French, Spanish and Italian editions have since been published. In the last decade Horrocks has written and drawn a wide range of projects including scripts for Vertigo's Hunter: The Age of Magic and the Batgirl series, and Atlas, published by Drawn and Quarterly.
Horrocks' work has been displayed at the Auckland Art Gallery and Wellington's City Gallery. In 2002 Hicksville won an Eisner Award for Talent Deserving of Wider Recognition, and the same year Atlas was nominated for the Harvey Award for Best Single Issue or Story in 2002. In 2006 he was appointed University of Auckland/Creative New Zealand Literary Fellow.
In an interview with Comics Bulletin, Horrocks claimed that his first words were 'Donald Duck'.