For a number of years she has produced the webcomic Moving Pictures with Stuart.
In 2007 she began working for Marvel Comics writing a Hellcat story, with Stuart providing the art, for the first four issues of Marvel Comics Presents. This was followed in 2008 by a five issue Patsy Walker: Hellcat mini-series, this time with artist David Lafuente.
In May 2009, Immonen became writer for the Marvel Comics' award-winning series, Runaways with artist Sara Pichelli.
Immonen and artist Tonji Zonjic are the team behind Marvel's new mini-series Heralds, featuring an all-female cast of main characters.