- With great muscles comes great responsibility.
- — Flex Mentallo src
At one point in his career, he fell in love and married a woman named Dolores. Flex kept no double identity, and was a well known figure in his hometown. One day, while on a picnic with Dolores, Flex heard a small boy crying for help and went to investigate. Discovering the boy, Flex saw the child's kitten was stuck in a tree and used his powers to get it down. Unfortunately, what had seemed to be a routine superhero moment was in reality a trap. Flex was captured by the Bureau of Normalcy and held prisoner for decades, restrained so he couldn't use his powers to escape.
After being sent to the Ant Farm, Flex met Larry Trainor and asked for his help in escaping. Instead, Larry adamantly refused, though the Negative Spirit left his body, surprising him. Flex and the Spirit engaged in a brief conversation, where Flex promised to take him back to his home if he aided in his breakout. However, Charles Forsythe and several agents of the Bureau entered his cell and tried to take him away. Flex used his powers to throw one of his agents across the room and stated that he was Flex Mentallo, not 722. Forsythe and the others then began torturing him. Later on, when Larry Trainor had returned following another session with Forsythe, Flex once again urged Larry to escape with him, this time in order to save the Negative Spirit. Larry doubled down on his choice, forcing Flex to endure another confrontation with Forsythe, where he repeated the same process, only this time he was shown a video of his wife Dolores being held hostage, infuriating him. Forsythe promised Flex that he'd be able to regularly see her, forcing him to resign his attempts at escaping.
Several decades later, Flex Mentallo, still residing in the Ant Farm had drastically changed, growing a thick beard and long hair as well as forgetting his own identity, going only by 722. Flex then befriended a new prisoner attempting to escape. Flex informed him that, by his experience, escaping wouldn't work and introduced himself as 722. He remarked on his new attitude toward the Ant Farm, having been serving his country in exchange for certain perks such as an armchair and a television, though he never remembers his time working for the Bureau. Upon being asked how long he'd spent in the Ant Farm, Flex struggled to remember and guessed it was around the 1960s. Flex later overheard Vic screaming as his A.I. kept suggesting him to run a diagnostic check and irritably told it that Vic didn't want to do so. Vic asked Flex how he could hear Grid though he refused to answer, instead wishing to continue watching his show.
When Vic's allies, the Doom Patrol, infiltrated the Ant Farm to rescue him, Flex was one of the many prisoners that found themselves released as a result. Rita Farr then remarked on how alike he was to the Flex Mentallo they're searching for only to come to conclusion that 722 was, in fact, Flex himself. Before the Bureau could surround the group, Jane's personality Flit teleported everyone to Doom Manor. Cliff Steele and the others debated on whether or not 722 was Flex, as he had no memories of being who they claimed he was. Cliff and Jane then cleaned up 722, shaving and cutting his hair in order to get Flex to remember himself. However, this didn't work, prompting them to try and get a rise out of Flex by continuously interrupting his favourite television show. Instead of triggering any latent memories, Flex simply broke down and began crying. Larry then arrived to suggest that they find his wife, Dolores, in order to bring his memories back. Flit then swiftly found and reunited her with Flex, allowing him to remember his old life. Unfortunately though, their reunion was short-lived as a safeguard built into Dolores activated, causing her to deteriorate and die in his arms. Enraged, Flex used his Muscle Mystery power, causing all the lights in the manor to be destroyed.
- Muscle Mystery: Flex Mentallo's powers derive from a strange energy source known to him as the "Muscle Mystery". Flex is capable of doing more or less anything by flexing his muscles.
- Flight: By wiggling his ears, Flex can grant anyone a limited amount of flight. 
- Dimensional Travel: By flexing his splenius capitis, Flex can transport a person wherever they're focused on going, such as within a comic book. 
- Energy Construct Creation: Flex can materialize almost anything by flexing his muscles. He was able to materialize a flower as well as a picnic set. 
- Pheromone Control: Flex can cause everyone in his surrounding area to experience an intense orgasm by flexing his splenius cervicis. 
- Telekinesis: By flexing his eye muscle, Flex could throw a man across a room with telekinesis. 
- This version of the character is exclusive to the continuity of the television series Doom Patrol and is an adaptation of Flex Mentallo. The original character was created by Grant Morrison and Richard Case and first appeared in Doom Patrol (Volume 2) #35.
- Flex Mentallo is portrayed by Devan Long.
- 4 Appearances of Flex Mentallo (Doom Patrol TV Series)
- 2 Images featuring Flex Mentallo (Doom Patrol TV Series)
- 1 Quotations by or about Flex Mentallo (Doom Patrol TV Series)
- Character Gallery: Flex Mentallo (Doom Patrol TV Series)