His works for 2000 AD include the miniseries Necronauts (2000–2001), in which Harry Houdini, Arthur Conan Doyle, Charles Fort and H. P. Lovecraft team up to defeat an alien menace that seeks to destroy the human race.
This was followed up by the series Caballistics, Inc., a story about a recently privatized team of occult researchers, which included a combination of pop-cultural references and labyrinthine conspiracies.
By 2004, Rennie had become a writer on the Judge Dredd strip, following up a number of subplots initiated by its principal author, John Wagner, as well as developing his own situations and guest characters, some of whom have spun off into stories of their own.
He also co-wrote The Doomsday Quatrain and sections of 1001 Nights with Emma Beeby for Big Finish Productions' Doctor Who main range of audio plays.
He is known for maintaining close links with the fan community, for example by contributing to small press comics such as Solar Wind, despite a public persona he describes as "a grumpy Scottish git".