Flex Mentallo is a fictional character appearing in American comic books published by DC Comics. 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.

Grant Morrison MBE is a Scottish comic book writer, screenwriter, and producer. Their work is known for its nonlinear narratives, humanist philosophy and countercultural leanings. Morrison has written extensively for the American comic book publisher DC Comics, penning lengthy runs on Animal Man, Doom Patrol, JLA, Action Comics, and Green Lantern as well as the graphic novels Arkham Asylum, JLA: Earth 2, and Wonder Woman: Earth One, the meta-series Seven Soldiers and The Multiversity, the mini-series DC One Million and Final Crisis, both of which served as centrepieces for the eponymous company-wide crossover storylines, and the maxi-series All-Star Superman. Morrison's best known DC work is the seven-year Batman storyline which started in the Batman ongoing series and continued through Final Crisis, Batman and Robin, Batman: The Return of Bruce Wayne and two volumes of Batman Incorporated. They also co-created the DC character Damian Wayne.