As an entry into comic writing Claremont was given the fledgling title, Iron Fist in 1974 that also teamed him with John Byrne for the first time. Len Wein then gave him the writing duties for the relaunched X-Men. Claremont also found narrative excuses to sideline Professor X, as one of the problems with the original X-Men series was that the Professor would appear at the end of the story to magically correct the situation no matter how dire.
During his years as X-Men writer, Claremont wrote or co-wrote many classic X-Men stories such as the “Dark Phoenix Saga”, and “Days of Future Past”. He also co-created numerous important X-Men characters, including Rogue, Psylocke, Shadowcat, Phoenix, Sabretooth, Mystique, Emma Frost, Rachel Summers, Mister Sinister, Captain Britain and Gambit. He also helped launch best-selling spin-offs such as New Mutants, Excalibur, and Wolverine. In 1991, however, he left Marvel over differences with the editorial staff. In 1985 Marvel launched an X-Men spin-off X-Factor that changed the Phoenix/Rachel Summers continuity that Claremont had established. Also Claremont was upset when Jim Shooter used Magneto as a conventional villain in the Secret Wars even though Claremont had been slowly rehabilitating the character.
In 2000 he returned to Marvel (as part of the company’s “Revolution” event) and wrote Uncanny X-Men and X-Men until he moved to X-Treme X-Men with penciller Salvador Larroca.
As of 2005, he is again writing Uncanny X-Men, X-Men: The End, and New Excalibur for Marvel Comics.
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.
Outside of comics, Claremont co-wrote the Chronicles of the Shadow War trilogy, Shadow Moon (1995), Shadow Dawn (1996), and Shadow Star (1999), with George Lucas. This trilogy continues the story of Elora Danan from the movie Willow. In the 1980s, he also wrote a science fiction trilogy about female starship pilot Nicole Shea, consisting of First Flight (1987), Grounded! (1991), and Sundowner (1994). Claremont was also a contributor to the Wild Cards anthology series.