Best known for his work on Superman: Secret Identity, Nextwave, Ultimate X-Men, The New Avengers and Ultimate Spider-Man. Stuart Immonen started out pursuing a career in art, studying at Toronto's York University.

In 1988, he published a series called Playground, a one shot tale written by Kathryn Kuder; it was his first published work. After that he worked with several smaller comic book companies including companies like Top Cow and Image Comics before being hired by DC Comics and Marvel Comics in 1993.

Immonen has drawn a variety of high-profile characters as Superman, Hulk, the Legion of Super Heroes, and the X-Men. His work includes titles such as; Ultimate Fantastic Four and Ultimate X-Men (54-65) with writers Warren Ellis and Brian K. Vaughan, as well as a 12-issue run on Nextwave.

In 2005, he published 50 Reasons to Stop Sketching at Conventions, a tongue-in-cheek handbook for convention etiquette, the series is an 80 page b&w pocketbook collecting the irreverently satirical series of webcomics, and it exposes the peculiarly strained relationship between creator and fan. Besides self-publishing, Immonen had also maintained a webcomic called Never as Bad as You Think which is co-authored by his wife, Kathryn Immonen. The series didn't focus on the traditional three-act structure story but rather a collection of character pieces that inter-connect throughout the series.

In 2006, Immonen was the new artist on Marvel Comics, Ultimate Spider-Man starting with issue 111 ending on issue 133; he then departed from the series and began working on New Avengers, starting on issue 55 through to 64.

In 2010 Immonen pencilled the first seven issues of the New Avengers as part of the Heroic age relaunch written by Brian Michael Bendis.

In 2010, Immonen released Moving Pictures another webcomic co-authored by Kathryn Immonen, which was later turned into a graphic novel like Never as Bad as You Think which is currently been published by Boom Studios . The story was set in the World War 2 era, concerning the widely reported Nazi pillaging of much of Europe's art collections and an atypical romance between two people riding that roller-coaster.

Immonen worked on the seven issue mini series Fear itself with Matt Fraction providing the art for the whole series.

Immonen pencilled issue #21 of Secret Avengers written by Warren Ellis released in 2012.

Immonen pencilled issue seven of Avenging Spider-man featuring She-Hulk it was written by his wife Kathryn Immonen.

Immonen pencilled a 10 page fight in Avx vs #1 which was a fight between Thing and Namor it was again written by his wife Kathryn Immonen. He then drew a one page story in Avx vs #6 which was a Avengers vs X-Men Science battle this was again written by Kathryn Immonen.

Immonen is now working on All New X-Men part of the Marvel Now! relaunch with Brian Michael Bendis the series will focus on the original five X-Men coming forward in time to the present post Avengers vs X-Men world.

Immonen is an acclaimed artist said to have a simple style but is very good at showing what is happening on the page with facial expressions and characters body languages. Immonen is also very good at drawing very varied subject matter from Nazis to Sea monsters as is shown in the Fear itself miniseries. Immonen is acclaimed for drawing very dynamic and fluid action scenes in the books he draws.