After D.C. Thomson launched Warlord, a successful war-themed weekly, Mills was asked in 1975 to develop a rival title for IPC.

Action's mix of violence and anti-authoritarianism proved controversial and the title lasted less than a year before being withdrawn in the face of media protests.

Mills contributed Ro-Busters, a series about a robot disaster squad, which moved to 2000 AD when Starlord was cancelled.

Ro-Busters was the beginning of a mini-universe of interrelated stories Mills was to create for 2000 AD, including ABC Warriors and Nemesis the Warlock.

In 1986 he edited the short-lived comic Diceman, which featured characters from 2000 AD in political satire regarding U.S. President Ronald Reagan.

The title lasted until 1991 and launched the careers of talents such as Garth Ennis, John Smith and Sean Phillips.

In 1991 Mills launched Toxic!, an independent colour newsstand weekly comic with a violent, anarchic tone, perhaps as a reaction against the politically worthy Crisis, and a creator-owned ideal.

In 2010 Mills adapted a story that had been started by him and Wagner for Doctor Who in the 1980s and was produced by Big Finish as "The Song of Megaptera".

In 2017 he wrote, with Kevin O'Neill, and published two novels, Serial Killer and Goodnight, John-Boy, part of a planned series of four books.

In 2019 Mills announced that he would publish a new all-ages science fiction anthology comic called Spacewarp, to be released in 2020, and that the artists would retain the copyright on their work.

As well as his influential role in creating and contributing to numerous of British comics, Mills has produced work in both America and Europe.