Hailing from Watford City, Noth Dakota, Marvin Flumm originally found work as a shoe salesman. After being identified as having the mutant power of telepathy, Mentallo was recruited by S.H.I.E.L.D as one of their psychics.
He first appears in Strange Tales #141.
Mentallo was unpredictable, and not entirely morally focused or driven. He strays several times to the bad teams despite being offered a place at SHIELD.
Major Story Arcs
He soon turned against his employers however and joined forces with the Fixer, with whom he has teamed up with on numerous occasions. He was later employed by HYDRA. During a brief period, Mentallo joined a version of the Brotherhood of Evil Mutants, called the Resistants, under the pseudonym Think Tank. Mentallo has been hired by numerous criminals such as MODOK, Red Skull, Justin Hammer and The Hood. Ret-con origin stories have shown Mentallo to be Hydra's go-to person for training their new recruits, such as Jessica Drew (Spider-Woman I) in psychic defense.
Mentallo is one of "The 198", the few mutants who retained their powers after M-Day.
After years of obscurity and inactivity, Mentallo got a big break, being hired by MODOK as one of "MODOK's 11" for a major weapons heist. During the planning stage, Mentallo is attacked and seemingly killed by teammate The Chameleon, who is actually a Super Adaptoid imposter. Mentallo hasn't resurfaced since, though due to the perception redirection aspect of his powers, he could potentially resurface at any time, playing the "oh, you only THOUGHT you'd set me on fire and left me to die" card; similarly to when psychic X-men villain Mesmero returned from the grave.
He joined the Hood's gang and recently fought Avengers Academy and was defeated. He is presumably now in prison
Mentallo, next shows up running Monster Island in the pages of Astonishing X-Men.
Powers and abilities
Mentallo possesses moderate psychic powers, allowing for such feats as mind reading. He usually has his powers amplified using devices. He also wears a battle suit and uses weaponry. In later years he's often used his powers to alter the perception of others, allowing him to evade attacks and pass through public places unnoticed.