Little was known of Leland's past before encountering the X-Men, however it is known that was a corporate lawyer, and that he encountered his later teammate, Emma Frost before she was inducted into the Hellfire Club. At the time, Emma was homeless and using her powers during a Hellfire Club meeting to gain information about stocks, Leland took an interest in her but came on too strongly, which made Emma run away. Shortly after, he accompanied Sean Cassidy and his NYPD partner to an incident behind the club which involved Emma. Emma, having met and repaired the fractured mind of the Dark Beast with her powers, mind-wiped all three men into forgetting the incident and that they had ever met.
Later, Leland first encountered the X-Men when they invaded the New York headquarters of the Hellfire Club. Leland caused Wolverine to become super-heavy and fall through the club's floor into a sub-basement. In the rematch, Wolverine attacked Leland from above. At that moment, Leland foolishly used his mass-increasing powers against his opponent a second time, resulting in Wolverine crashing through the floor again, only this time with Leland beneath him as Wolverine intended. All evidence to the contrary, Leland somehow survived this incident.
Alongside most of the rest of the Hellfire Club's Inner Circle, Leland later battled the X-Men in New York's Central Park. Leland used his power to sink Colossus hundreds of feet underground. When the robot mutant hunter Nimrod attacked the assembled mutants, the X-Men and the Club joined forces to battle Nimrod. Against considerable resistance by the robot, Leland increased Nimrod's gravity to move it towards the ground and thus vulnerable to attack. Overweight and in poor health, the effort caused Leland to have a heart attack. Then, at the X-Men's leader Storm's urging, Leland increased the gravity of his teammate Sebastian Shaw (who had been sent hurtling towards outer space by Nimrod), causing Shaw to crash into Nimrod's body on the ground. Having accomplished this task, Leland succumbed to his heart condition and died.
At a later date, the supervillain Black Talon revived Leland's body, as well as the bodies of three other deceased mutants, as a zombie; his motives for doing such were unspecified. After She-Hulk's defeat of the Talon and his X-Humed, Leland was reburied. Precautions were taken to ensure that he never come back, among them filling his mouth with salt and sewing his lips together.
Nevertheless, Leland was resurrected again by Selene Gallio and Eli Bard using the Transmode virus. When Selene was killed, Leland's body cease to be.
X-Men: The Animated Series
Harry Leland figured prominently in the Dark Phoenix saga in the X-Men animated series.
Wolverine and the X-Men
Harry Leland is featured in Wolverine and the X-Men. He is seen as a member of the Hellfire Club trying to attain the powers of the Phoenix from Jean Grey.