It's widely accepted that only the worthy are able to wield the Hammer of Thor, Mjolnir. However, what about exceptions to the rule given any extenuating circumstances? For instance, can Magneto, mutant arch-nemesis to the X-Men and Master of Magnetism, wield Mjolnir?
There's been those who have used the hammer beyond the Son of Odin, such as Captain America wielding Mjolnir, both in the comics and on-screen, or with Beta Ray Bill, the alien Korbinite ally of Thor. Both of them have been considered worthy, so the rule stands. However, does Magneto break that rule, given his powers and control over metal?
The question of whether or not Magneto would be able to wield the hammer on worthiness alone is simple. He's not. He's traditionally been one of the biggest and baddest villains in the Marvel Universe, willing to do whatever it takes to see his vision of mutant superiority over humankind a reality. Even with his moral compass turning a little more altruistically in recent years, he still wouldn't be worthy to lift Mjolnir. Case closed right? Unfortunately it's not so simple when his powers are factored in.
Magneto's powers provide some alternative angles that might let him serve as the exception to the worthy rule. Firstly, Magneto's manipulation of metal seems to have little limitations. Both on the comic book page and on the screen, Magneto's been able to easily lift and control large and heavy metal objects for a variety of goals and actions. Why not Mjolnir? The hammer is made from Uru, a metal forged by dwarves in the heart of a dying star, Nidavellir. Even so, space metal is still metal, meaning he should be able to control the Uru in the hammer...maybe. He could still be prohibited by his lack of worthiness, especially if the magical rule trumps Magneto's genetic ability.
There have been times where Magneto has sort of wielded the hammer in the comics. In the Ultimate universe, Magneto gets a hold of the hammer by manipulating the electromagnetic field around the hammer, thus using it but not technically wielding/holding it, circumventing any need to be worthy. However, that is in the deviating and separate Ultimate Marvel universe. Whose to say if the same method would work in the standard 616 universe? Furthermore, Magneto can at least stop the hammer's trajectory after it's been thrown. In Journey Into Mystery #109 from Stan Lee and Jack Kirby, Magneto stops it while it's in midair.
So, can Magneto wield Mjolnir? No. Could he potentially control it under certain conditions? Yes. To have the hammer in his hand wouldn't and shouldn't ever happen. He's just not worthy of it under the magical conditions set on the hammer as per Odin The Asgardian All-Father. However, his genetic power and ability to control metal and magnetic fields would give him a leg up in countering its power. At least more so than most other villains and foes. In any case, it doesn't seem like Thor needs to worry about Magneto swiping his hammer any time soon.