Created by writer Grant Morrison and artist Richard Case in 1990, during their run on Doom Patrol, Flex is in part a parody of Charles Atlas' long-running "The Insult that made a Man out of Mac" advertisements seen in American comics from the past.

In 1996, Flex Mentallo appeared in a self-titled, four-issue miniseries written by Morrison and illustrated by Frank Quitely.

[1] Mentallo appears in his first live-action adaptation on the Doom Patrol television series for DC Universe and HBO Max, portrayed by Devan Chandler Long.

Flex Mentallo first appeared in 1991 in issue #35 of Grant Morrison's run on Doom Patrol as a member of the audience for Danny the Street's Perpetual Cabaret.

In issue #42, Flex is revealed to be not an entirely original creation, but rather a parody of the post-workout protagonist of Charles Atlas' long-running "The Insult that made a Man out of Mac" advertisements seen in American comics from the past.

Part of a long string of fictional characters "come to life" in Morrison's writing, Flex was apparently created by a psychic child with a green pen.

Charles Atlas company president Jeffrey C. Hogue was unhappy with its likeness being used this way, and filed a trademark infringement suit against DC Comics.

[2] DC has reprinted Flex's initial appearances, including the Charles Atlas parody, in the third and fourth collections of the Morrison Doom Patrol stories, "'Down Paradise Way" and "Musclebound".

When the false Men from N.O.W.H.E.R.E., controlled by a fanatic named Darren Jones, tried to destroy Danny the Street, the Doom Patrol came in to try and put a stop to it.

Harry was "on to something big", as Norman put it, involving the Pentagon, a rash of 32 mysterious disappearances since 1949, silver sugar tongs, and an Ant Farm.

Upon Wallace's death, Flex regained his powers and successfully transformed the Pentagon into a circle, but he couldn't destroy the Avatar.

Flex Mentallo joins up with Doom Patrol later, helping them solve problems in the realm that is deep inside Danny.