Seymour Gutz was originally a nerdy delivery man for the Fly-by-Night Messengers Service, who suffered an unfortunate syndrome of disorientation, which hampered him at his work a lot. In one of his usual aberrations, Seymour accidentally stumbled into a secret laboratory of the Shredder's where Krang's special Mutagen stood boiling ready for pouring into a rocket's fuel tank that would be distributed by the rocket's vapor trail during takeoff all over New York City, turning every human being into a mutant. Mutagen Man was created when Rocksteady and Bebop were happy to shoot at the unwanted intruder causing him to topple from his catwalk into the underlying vat of Mutagen.
The mutagen that Seymour had fallen into was so unstable that it dismembered Seymour's body into his somehow still living brain, his eyes, his mouth and his spinal cord. In order to survive, the Shredder, Rocksteady and Bebop built Seymour a special teal and purple containment suit that he was forced to wear, whose torso contained a tank of mutagen that preserved the remains of his body until another resolution could be found. By chance, Seymour found out shortly thereafter that due to his condition, he could transform into almost any form if only temporarily. However, he could not turn into his old self again.
He did not like this and Shredder took advantage of this by promising to turn Seymour back into his old self permanently if he would get him a canister of Byndex-3, a substance that he and Krang needed to make permanent the effects of the mutagen in the rocket fuel. Mutagen Man agreed to this and disguised himself as a security guard, Irma, Michelangelo, an old lady, and Vernon Fenwick. As a result, Seymour came into conflict with the Turtles and April O'Neil, but after the Shredder cheated him, Seymour helped the Turtles to thwart the Shredder and Krang's plan. Donatello eventually cured Seymour of his mutation by pouring the Byndex-3 into his suit, in order to stabilize his body and having him choose a final form. Seymour Gutz reverts to human form, but chooses the appearance of a good looking man. When asked why he doesn't look anything like his photo, Seymour responded with "I may be a nerd, but I am not stupid." He then finishes the job he was required to do: deliver a letter to the Touch of Romance Flower Shop. He ends up going on a "night on the town" with April O'Neil.
- Mutagen was based on a sketch made by Peter Laird for Playmates Toys, Laird originally called it "The Unknown" and would be a mutant crossing between an ape and some kind of insect that went wrong.
- His human name is a pun "see more guts" due to the fact that his organs are visible.