When Jean Grey's mutant telepathic powers emerged, the voices and visions nearly drove her insane. She had been committed to a psychiatric institution when fellow psi Charles Xavier saved her and taught her to control her powers. Recruited in to Xavier's X-Men as Marvel Girl, she in turn enlisted many others. Her teammate Scott Summers (Cyclops) was attracted to her, but too uncertain to act; though Jean knew this, she instead became involved with new recruit Wolverine. When she learned he had originally joined intending to kill Xavier on behalf of the Brotherhood of Mutants, she dumped him. The X-Men were subsequently abducted by Weapon X, Wolverine's former employers. After six weeks in their brutal custody, Jean gained a new perspective on her former lover and forgave him, though they did not get back together. On her first mission, Jean was blackmailed in to murdering Indian geneticist Atul Pandya to prevent Weapon X from killing Cyclops; in spite of this, when the X-Men were freed by the Brotherhood, Jean convinced her teammates not to slay their former captors.

Jean finally made the first move on Summers and they became an item. Around the same time, her seizures returned and she experienced visions of a giant burning phoenix. She was convinced she had contacted a hostile extra-dimensional entity. This belief was shared by the Hellfire Club, Xavier's mysterious financial backers, who carried out a mystic ceremony to merge Jean with the "Phoenix God", hoping to precipitate the end of the world. The ceremony appeared to work, but instead of granting the Club power, the possessed Jean slew them all. Xavier, insisting she was merely delusional, managed to restore Jean's own personality. No longer overwhelmed by the Phoenix power, Jean continued to access it, once lifting an unstable nuclear reactor in to the atmosphere and atomising it, thus saving the East Coast from devastation. Confident she could control the power, Jean believed the Phoenix had been an illusory manifestation created by her inability to deal with her own immense powers. Thor of the Ultimates advised caution, saying that if he were trying to possess someone, he would try to convince the host he did not exist.

Grey has continued to participate in various X-Men missions: bringing the explosive mutant Geldoff to the Mansion for treatment; trying to track serial killer Sinister by communicating telepathically with surviving victim Jean-Paul Beaubier; locating Rogue when she was kidnapped by Gambit; and unintentionally and temporarily swapping Wolverine's mind with Peter Parker's (Spider-Man). Jean recently learned that Cyclops lied when he told her that she was his first girlfriend.

Recently, an interview was set up by Professor Xavier for Jean to undergo testing with Lilandra, who was the leader of the Shi'ar Church of Enlightenment. Jean was asked questions about her childhood and her current life with the X-Men. The questions continued to become more personal and Jean was visibly stressed by the interrogation. When asked a question about her parents, the Phoenix flared up inside Jean and took control. She sought to destroy the world, starting with Lilandra. Professor Xavier tried to temporarily shut down Jean's brain using his psionic powers but failed. As the Phoenix was seemingly going to kill Xavier and Lilandra, the Professor muttered the words, "I love you," to her, which made her vulnerable for a short time. Xavier took the opportunity and shut her brain down. When Gerald, Lilandra's assistant, returned he stated that Jean was not the reincarnation of the Phoenix; rather, her powers mimicked the actual Phoenix because Jean's parents had once been a part of the Shi'ar Church of Enlightenment. When Jean awoke in her room she had no memory of these events.

Professor Xavier decided to confine Jean to a cell in the sub-basement of his mansion as he did not believe her to be able to control her powers. However, she subsequently broke out of her cell to pursue the Magician, whom she believed had taken her place within the X-Men.