Following the events of X-Men, Magneto is incarcerated in a prison designed specifically to not contain any metal whatsoever. Mystique seduces Magneto's prison guard Mitchell Laurio while he's off-duty and injects him with liquefied iron. When Laurio returns to work the next day, Magneto kills him by extracting the iron from his blood. He then shapes it into several marbles and uses them to shatter the prison and free himself.
Before lifting of RFK Stadium, Magneto attempts to retrieve his stolen helmet. Possessing two steel marbles as weapons, he enters the Pentagon, knocks out two guards and a clerk, and retrieves his lost helmet.
- Unlike X2, where Magneto extracted the iron from Laurio's bloodstream in order to form the marbles, it is unknown how Magneto came across the marbles he possessed in Days of Future Past. They could've either been formed or stolen.