In 1988 Mitchell began writing in partnership with Pat Mills, who met the writer while Mitchell was working as a shop manager for Acme Comics in Coldharbour Lane in Brixton, South London. Mills was looking for a black writer to help him create a nightmare urban world based in the UK. This would complement the one that Mills had developed with his main character Eve and her friends in Central America with a focus on corporate exploitation by the multinationals in the third world. It was the beginning of a writing partnership that would last until 2004.
In Crisis, the revolutionary political comic from Fleetway, Mitchell worked on Books 2 and 3 of the controversial story, Third World War. This was a complex and hard hitting narrative that covered issues including matriarchy, police racism, no-go areas, private police forces, class war, and black resistance (Newsinger, 1999). The stories anticipated the surveillance society and Macpherson by at least a decade. Mitchell also had the opportunity of writing an Amnesty International story "Prisoner of Justice" with Glenn Fabry as artist. Amongst the most memorable Third World War' stories were "Liat’s Law" parts 1&2 with artist Duncan Fegredo, and "The Black Man’s Burden". This classic quartet of stories, with John Hicklenton's art, introduced the character of the villainous Chief Inspector Ryan, the embodiment of racism within the police force (Newsinger, 1999). The tales provided the platform that Mills had framed for Mitchell to express his political perspective and cultural concerns of the time. The Black African Defence Squad (BADS), and the mothers of Azania, Sonnyboy and Charles Shebego amongst a number of other characters, served to develop a complex and arresting depiction of black African urban culture in comics. Sean Phillips was the other major artist who collaborated on a number of framing episodes. (Newsinger, 1999)
Mitchell went on to partner with Pat Mills on Toxic! on which he co wrote Coffin for that publication. He also worked for the well-regarded black community magazine The Alarm through the mid 90’s, writing the comic strip The Scrolls of Imhotep amongst other pieces.
Mitchell has gone on to write several horror shorts for various American publishers chiefly Lament for Baal with artist Kane: 1327 for Raw Media Magazine. He has also worked on a number of uncredited stories with Pat Mills including, ABC Warriors and Judge Dredd in 2000 AD, and Star Wars for Dark Horse. Finally he partnered Mills in the first ABC Warriors novel The Medusa War for Black Library based on elements changed or removed from the scripts. According to Mills:
Mitchell has just completed the first episode of RaptorZ a new internet comic with Glenn Fabry as artist for Buzz Comic (http://www.buzzcomic.com) that will be appearing in the summer 2010. He is also working with the artist Kane: 1327 on Fierce, a horror anthology due for completion winter 2009.