- "I've always been interested in how the antagonists and villains work, and what makes people bad, and all these kinds of things."
- ―Kieron Gillen
Gillen's first comic-book project was the 2006 Image Comics series Phonogram, drawn by regular collaborator Jamie McKelvie. In 2009, Gillen began writing for Marvel Comics. He has written for Thor (volume 3), Uncanny X-Men (volumes 1 and 2) and Iron Man (volume 5). His most popular work for the company was Journey into Mystery, focusing on the misadventures of Kid Loki, and the subsequent Young Avengers (volume 2), drawn by McKelvie. Gillen and McKelvie are currently collaborating on another creator-owned comic published by Image, The Wicked + The Divine.
Star Wars: Episode V The Empire Strikes Back was the first film Gillen ever saw at the cinema, and he compared its impact on him to that of Tolkien. As a journalist, Gillen wrote a highly enthusiastic review of Star Wars: Knights of the Old Republic, which he described as reigniting his love of the saga, having played mediocre Star Wars games that were given more positive write-ups for simply being Star Wars, or fans who were "cursing George Lucas' name only to—a handful of months later—go out and buy the DVD anyway 'for the extras.'" He later wrote, "Star Wars isn't great because it's Star Wars. Star Wars is great because it is Great. A Star Wars that isn't Great isn't Star Wars in any meaningful way."
On January 9, 2019, Gillen revealed that after the May 2019 release of the sixty-seventh issue of the Star Wars comic-book series, he would leave the Star Wars universe to embark on other projects because he had told all the stories he desired.
- "Launch Pad"—Star Wars Insider 152
- "Blaster"—Star Wars Insider 155
- "Marvel Comics 2015: A Marvel Galaxy"—Star Wars Insider 158
- "Blaster"—Star Wars Insider 158
- "Breathing New Life Into a Dark Lord"—Star Wars Insider Special Edition 2016
- "Launch Pad"—Star Wars Insider 167
- "Blaster"—Star Wars Insider 167
- "Sith Star"—Star Wars Insider 169