First Appearance: X-Men #1 (September 1963).
Appearances: Far too many others to list.
Years Active: Late 1940s-present.

One can't exactly call Magneto a hero by any means. But according to the published stories about his early history, his powers manifested themselves after he left the death camps, and he spent the late 1940s and the 1950s hunting down German war criminals and being active on the side of right. We know he was active along with Charles Xavier against Baron Strucker in Israel in the late 1950s or early 1960s. And it has been established that at some point after he helped Xavier against Strucker Magneto did some work for the CIA under the codename "Magneto," hunting down fugitive Nazis. So I think he fits here. (His turn to evil and humanophobia is traced to his CIA handlers' decision that Magneto must be killed, so they attacked him and killed his lover.)