Since the first mutants were introduced in 1963's X-Men #1, they have primarily occupied their own corner of the Marvel Universe. As the X-Men have expanded into multiple spinoff teams, who have each battled their own collections of rival mutant enemies, it has only been on significant occasions that Marvel's mutants have interacted amongst the larger superhero community.
While most of these instances have happened in the form of team-ups or misunderstandings featuring the larger X-Men family, there have been a handful of instances in which established mutant heroes have become fully-fledged members of otherwise non-mutant teams.
Firestar - New Warriors
First introduced to the world in cartoon form as one of Spider-Man's Amazing Friends, Angelica Jones, better known as Firestar is a mutant with the ability to control microwave radiation, enabling her to fly and generate intense waves of heat. Her comic book version debuted as a member of The Hellions, the Hellfire Club's teenage rivals to Xavier's New Mutants.
After a short tenure with this group and the cancellation of the cartoon, Angelica was mostly forgotten for the next half-decade, before reappearing in 1990 as a founding member of the New Warriors. She served as the primary mutant representative of The New Warriors for their entire 75-issue original run.
Northstar - Alpha Flight
The smug super-speedster Northstar is best known today as a frequent member of the X-Men, but he was actually around in the comics for over two decades before joining their ranks. First recruited as an Economics teacher at the Xavier Institute, he was later conscripted into active duty and has been with the team ever since.
Prior to his recruitment, however, Northstar was a longstanding member of Alpha Flight, the Canadian government's answer to The Avengers. One of the few mutant heroes to debut as a member of an otherwise non-mutant team, Northstar's route to the X-Men was gradual, but he has made up for lost time with his unwavering support for their cause.
Angel & Iceman - Champions & Defenders
With the original X-men disbanding following the introduction of the "All-New, All-Different" team, Angel and Iceman first landed in Los Angeles, co-founding the short-lived superhero team The Champions. After this brief but memorable stint, they would later be recruited together as freshman members of a newly reorganized Defenders, where they were also reunited with former X-Men teammate Beast.
As the traditional non-team attempted to become more official, Angel's then-girlfriend Candy Southern would become their de facto team leader, while Angel himself became the primary financier of the organization. Iceman's membership was less notable, but he nonetheless continued to serve alongside his fellow Xavier's graduates until the series' eventual cancellation.
Beast - Avengers
The first of Xavier's students to graduate, Beast joined The Avengers shortly before the X-Men's 1975 relaunch. Prior to this, the X-Men comic itself had spent the past five years reprinting old stories while frequently facing potential cancellation, leaving the original characters on the table for use in other series.
Despite being the brains of the X-Men, Beast's intelligence took a backseat to established Avengers Iron Man and Henry Pym, and he instead found his niché as the team's comedic relief alongside his teammate, the recently redeemed villain Wonder Man. While technically a member of The Avengers, Beast continued to be pulled into the X-Men's mutant-centric adventures, including a four-issue team up with Jean Grey before later playing a prominent role during the infamous Dark Phoenix Saga. After an adventure alongside Doctor Strange, Beast would resign from Avengers membership, choosing instead to help reorganize The Defenders into a more formal team.
Storm - Fantastic Four
Following Marvel's Superhero Civil War, which saw The Fantastic Four's Reed and Sue Richards aligned with opposing sides, the couple decided to take some time off to repair their damaged relationship. Their hand-picked replacements were the X-Men's Storm and her husband, T'Challa, King of Wakanda, better known as The Black Panther.
While T'Challa's intelligence and Storm's raw power proved a suitable substitute for the roles of Reed and Sue respectively, the couple spent their tenure frequently divided between their adventures with the Fantastic Four, Storm's dedication to her fellow mutants, and T'Challa's responsibilities as the leader of an entire nation.
Havok - Starjammers
Alex Summers took over leadership of the spacefaring Starjammers following his father Corsair's death at the hands of the youngest Summers sibling, Vulcan. Alex had initially chased his previously unknown brother into space alongside a ragtag group of new and classic X-Men during the epic Rise And Fall Of The Shiar Empire storyline, the conclusion of which saw the death of not only Summers patriarch Corsair, but the Shiar Empress Lilandra as well.
After marrying Lilandra's sister Deathbird, Vulcan capitalized upon this power vacuum and assumed control of one of Marvel's most dangerous alien empires. Havok and his X-Men teammates Marvel Girl and Polaris, who also joined The Starjammers at this time, were labeled outlaws by this new regime, as they bounced around the galaxy launching coordinated attacks in an effort to avenge Corsair and bring Vulcan to justice.
Sunspot - A.I.M.
An original member of The New Mutants, Roberto Da Costa, aka Sunspot, has since been a member of numerous other superhero teams, including X-Force and The Avengers. His most unexpected affiliation however is with the renegade terrorist cabal of scientists Advanced Idea Mechanics.
Using his vast family fortune to outright purchase the organization, Da Costa hoped to use their resources to investigate the "incursions" that were rippling throughout Marvel's multiverse, systematically eliminating variant worlds. Within eight months of taking over the operation, Sunspot had cleaned house, reestablishing A.I.M. as a scientific superhero support group and rebranding them as "Avengers Idea Mechanics."
Kitty Pryde - Guardians Of The Galaxy
Tenured X-Man Katherine "Kitty" Pryde and Guardians leader Peter "Star-Lord" Quill began a long-term relationship after meeting during a team-up between their respective allies. When Quill was abducted during an intergalactic facetime with Kitty, she stole an Avengers Quinjet and set off into space to rescue him.
Following this adventure, Peter proposed, and Kitty accepted, deciding to leave the X-Men to stay in space with her new fiancé. After a few months by Quill's side, Kitty became disillusioned when his responsibilities as the newly appointed King Of The Spartax Empire put a strain upon their relationship. After a falling out with Quill, Pryde joined the Guardians Of The Galaxy full-time, assuming her estranged fiancé's former identity and becoming the new Star-Lord.
Wolverine - New Avengers
When writer Brian Bendis set about his long-term plan to disassemble, then reassemble Earth's Mightiest Heroes, his mission statement for the reformed group was to make it more akin to competitor DC's Justice League, with a team featuring Marvel's most recognizable established heroes. As such, perpetual Marvel mainstays Spider-Man and Wolverine were each granted first-time Avengers membership within the ranks of the relaunched New Avengers comic.
While an active Avenger, Wolverine additionally maintained his X-Men status, as well as continuing his own solo adventures and later forming a new version of the covert mutant team X-Force. As one of the publisher's inarguably most prolific characters, Wolverine's status as a member of the world's premiere superhero team was a long time coming.
Professor X - Illuminati
Following the events that drew Earth's heroes to become involved in the ongoing war between the alien Kree and Skrull empires, the leaders of Marvel's superhero communities began organizing in secret, hoping to intercept future threats before they could become as severe. Referred to as The Illuminati, this group consisted of the armored Avenger Iron Man, Inhuman King Black Bolt, Reed Richards of The Fantastic Four, King of Atlantis Namor, Sorceror Supreme Doctor Strange, and X-Men leader Charles Xavier. In addition to representing earth's mutant community, Xavier was tasked with telepathically monitoring each member during meetings, to ensure that none were attempting to betray or mislead the others.
While well-intentioned, the proactive approach of The Illuminati inevitably caused more harm than good. During a mission to the Skrull homeworld, ostensibly to inform them that the Earth was off-limits, the entire team was captured. After analyzing the DNA of this collection of powerful heroes, the Skrulls were able to lay the groundwork for the advanced technology that eventually allowed them to infiltrate Marvel's superhero community during the epic Secret Invasion storyline.