Marvin Flumm was born in Watford City, North Dakota. He once worked as a shoe salesman. As a mutant telepath of moderate ability, Mentallo was recruited by the original S.H.I.E.L.D. as a candidate for their ESP Division. However, he teamed with the Fixer in an attempt to take over S.H.I.E.L.D., clashing with Nick Fury for the first time. He then served as a division leader in the HYDRA fragment led by Silvermane. Mentallo teamed with the Fixer again, and they placed Deathlok (Luther Manning) under their control. They attempted to make the U.S. President their slave, but were thwarted by the Fantastic Four. Mentallo and the Fixer were employed by HYDRA again, but Mentallo was rendered comatose by the Micronauts. In Professor Power's custody, Mentallo battled Professor X on a psychic plane. Mentallo was captured by the Super-Adaptoid (impersonating the Fixer) but he was rescued by the Avengers.