A magnetic-powered mutant (or "post-human"), Erik Lensherr approached mutant telepath Charles Xavier after hearing of his work saving young mutant patients with uncontrollable powers. Lensherr and Xavier found each other fascinating, neither having met an adult mutant before. They became close friends and developed an ideology regarding mutants as mankind's replacements. Eventually, both men left their wives to found the Brotherhood of Mutants, a safe haven for persecuted mutants. Erik took his mutant children with him, and would later deny he ever loved his human spouse. A few years later, the Brotherhood moved to a remote Pacific island, the Savage Land, to found a new civilization. Erik even devised a new language for this new world, and through genetic experimentation created dinosaurs to inhabit it. Increasingly convinced humanity would not peacefully relinquish the planet to Homo Superior, Erik became more radical, regarding the Savage Land's citizens as an army. Abandoning his human name, Erik took the title Magneto. Realizing that Xavier didn't agree with his views, a paranoid Magneto took to wearing a helmet to prevent psi intrusion into his mind. Xavier finally fled with a handful of followers, but Magneto decided to teach him a lesson and broke his spine, leaving him crippled. Soon after, Magneto led the Brotherhood in an anti-human campaign of political assassinations and terrorist bombings.

Following an attack on Washington, the U.S. government unleashed robotic Sentinels on the mutant population in retaliation. When Xavier's new students, the X-Men, interfered, Magneto learned Xavier had survived, and sent Wolvrine to slay his former friend. He also had the President's daughter kidnapped, which halted the Sentinel campaign; the X-Men soon rescued her. American authorities located the Savage Land and launched a massive Sentinel attack, but Magneto used his powers to reprogram them, leading them back to assault Washington. While the Sentinels decimated the capital, Magneto dragged the President naked on to the Whitehouse lawn. The X-Men intervened, and Magneto was defeated when his son Pietro, convinced that genocide was going too far, removed his helmet, leaving him vulnerable to Xavier's powers. Xavier claimed to have killed Magneto, but had instead secretly brainwashed him, hoping to rehabilitate him. For several months the amnesiac Erik lived peacefully as a Manhattan social worker looking after disabled children; however, the Brotherhood eventually learned he was alive, and restored his memories. Magneto resumed the bombing campaign halted by his errant children, simultaneously gathering every mutant he could into a floating Arctic Citadel. His powers enhanced by a machine of Forge's design, Magneto intended to reverse Earth's magnetic field, leaving humanity to perish in the ensuing environmental chaos. The X-Men stopped him, and he was imprisoned in the Triskelion, home of S.H.I.E.L.D. and the Ultimates. He was recently freed by Mystique and Forge.