Claremont is best known for his 16-year stint on Uncanny X-Men from 1975 to 1991, far longer than that of any other writer,[3] during which he is credited with developing strong female characters as well as introducing complex literary themes into superhero narratives, turning the once underachieving comic into one of Marvel's most popular series.

[7] During his tenure at Marvel, Claremont co-created numerous X-Men characters, such as Rogue, Psylocke (Betsy Braddock), Kitty Pryde/Shadowcat, Phoenix, the Brood, Lockheed, Shi'ar, Shi'ar Imperial Guard, Mystique, Destiny, Selene, Reverend William Stryker, Lady Mastermind, Emma Frost, Tessa, Siryn, Jubilee, Rachel Summers, Madelyne Pryor, Moira MacTaggert, Lilandra, Shadow King, Cannonball, Warpath, Mirage, Wolfsbane, Karma, Cypher, Sabretooth, Empath, Sebastian Shaw, Donald Pierce, Avalanche, Pyro, Legion, Nimrod, Gateway, Strong Guy, Proteus, Mister Sinister, Marauders, Purifiers, Captain Britain, Sunspot, Forge, and Gambit.

Claremont wrote many classic stories, including "The Dark Phoenix Saga" and "Days of Future Past", on which he collaborated with John Byrne.

X-Men #1, the 1991 spinoff series premiere that Claremont co-wrote with Jim Lee, remains the bestselling comic book of all time, according to Guinness World Records.

Alienated by the sports-oriented suburbs,[5] his grandmother purchased for him a subscription to Eagle when he was a child, and he grew up reading Dan Dare, finding them more exciting than the Batman and Superman comics of the 1950s and early 1960s.

As an entry into regular comics writing, Claremont was given the fledgling feature "Iron Fist" in Marvel Premiere as of issue #23 (Aug. 1975).

"[19] Claremont introduced new supporting characters to the X-Men series including Moira MacTaggert in issue #96[20] and Lilandra Neramani in #97.

[26] Comics writers and historians Roy Thomas and Peter Sanderson observed that "'The Dark Phoenix Saga' is to Claremont and Byrne what the 'Galactus Trilogy' is to Stan Lee and Jack Kirby.

[30] With artist Walt Simonson, Claremont produced The Uncanny X-Men and The New Teen Titans in 1982, an intercompany crossover between the top-selling Marvel and DC titles.

[38] Claremont's stories for Marvel Team-Up included the cast of NBC's Saturday Night Live appearing in issue #74 (Oct. 1978)[39][40] and the introduction of Karma, a character that later joined the New Mutants, in #100 (Dec.

[42] Claremont co-created numerous other important female X-Men characters, including Rogue,[43] Betsy Braddock,[44] Mariko Yashida,[45] Kitty Pryde/Shadowcat, Phoenix, Mystique, Lady Mastermind, Emma Frost, Siryn, Rachel Summers,[26] Madelyne Pryor,[46]: 211  and Jubilee.

[46]: 241  He co-created such notable male characters as Sabretooth, Pyro, Avalanche, Strong Guy, Captain Britain,[47] Forge,[46]: 218  Mister Sinister, and Gambit.

[56] In 1994, he wrote issues #10–13 of Jim Lee and Brandon Choi's series, WildC.A.T.s at Image Comics, in which he finally introduced his creator-owned character, Huntsman.

Claremont's run on Fantastic Four would last for 29 issues and 2 annuals, making it his longest tenure on a title outside of the X-Men.

[59] Despite outselling books such as Amazing Spider-Man and Batman,[60] Claremont was fired by editor Bobbie Chase in early 2000, a decision that he was angered by.

In 2004, Claremont was co-writer on JLA issues #94–99, the "Tenth Circle" story arc for DC Comics, which reunited him with his former Uncanny X-Men artist John Byrne, with Jerry Ordway as inker.

According to writer/editor Paul Levitz, Claremont's complex story structures "played a pivotal role in assembling the audience that enabled American comics to move to more mature and sophisticated storytelling, and the graphic novel.

The classic Claremont pose is either a character, head hung in shame with two enormous rivers of tears running down the cheeks as he or she delivers a self-loathing monologue, or a character with head thrown back and mouth open in a shout of rage, shaking tiny fists at heaven and vowing that the whole world will soon learn about his or her feelings."

Claremont made a cameo appearance in the opening scene of the 2006 film X-Men: The Last Stand, for which he is credited as "Lawnmower man".

He made a cameo appearance as a Congressional committee member alongside fellow comic book writer Len Wein in an early scene in the 2014 film X-Men: Days of Future Past.

[8] In July 2011 Claremont signed a deed of gift to Columbia University's Rare Book & Manuscript Library, donating his archives of all his major writing projects over the previous 40 years to the Library's nascent comics archives collection, forming the foundation of a research collection focusing on New York City-based cartoonists and comics writers.

Following the examination and processing of the materials, which include notebooks, correspondence, early story drafts, plays, novels, comic books and materials from Claremont's early training in the theater and his career as an actor, Claremont's archive will be housed at the Rare Book & Manuscript Library in Butler Library, separate from the graphic novels collection, and will be open to anyone who demonstrates a need for its use, with a Finding Aid being made available online.

Claremont's longtime editor Louise Simonson said, "his papers will provide many clues, not only to the evolving way comic books are created and presented, but also how they are perceived, licensed, bought and sold in America and around the world.

"[4] A symposium called Comic New York was held March 24–25, 2012 at Columbia University's Low Memorial Library to mark the occasion, which was organized by Graphic Novels Librarian Karen Green, Professor Jeremy Dauber, and editor Danny Fingeroth.