He co-created numerous characters for Marvel Comics including Captain Britain, Shadowcat, the New Mutants, Sabretooth, and the Phoenix. He has also co-created many best-selling books such as New Mutants, Excalibur, and Wolverine.
Claremont has written many stories for other publishers including the Star Trek Debt of Honor graphic novel, Sovereign Seven for DC comics and Alien vs Predator for Dark Horse Comics.
In 2000 he returned to Marvel and wrote The Uncanny X-Men and its spin-off book X-Men until he moved to his own book, X-Treme X-Men, with penciller Salvador Larroca.