Plus, he doesn't have much issue using whatever metal is near him. I think he bent a brass fountainpiece once, even though brass isn't magnetic. And the amount of control he can exercise seems like it's more than simple magnetism
The hammer is made from Uru, an Asgardian (metal? rock? No one really knows.). Uru is unique in that it absorbs energies, mostly magical, but it can absorb other types like radioactive, or electrical. So we can therefore assume that if Thor's hammer was to absorb electrical energy, which we often see it do when he calls down lightning, it would become magnetic, allowing Magneto to move it.
However, we must also consider the nature of Mjolnir itself. Mjolnir is a sentient fixed quantum point. That means that Mjolnir is a stubborn little shit who won't move unless it deems you worthy. So we must then ask the question: to Mjolnir, is Magneto worthy?
Obviously.
Who knows.