He is most known for his work on Thor, which he wrote and illustrated for nine years. In addition to his work for both Marvel and DC, Simonson has also generated a considerable body of creator-owned work, like Star-Slammers and Ragnarök. Apparently, there was a rumor that Simonson kept a list of Dr. Doom appearances that should be ignored due to mismanagement of the character by various writers. However, Brian Cronin of the Comic Books Should Be Good! podcast discovered that this rumor was unfounded. It is true, however, that Simonson is married to Louise Simonson. The couple made a cameo appearance in the cinematic Thor.
Simonson has yet to retire, and as of 2018, he continues to produce new work.