Megatox was once a scientist working for Dr. Robotnik on the Mega Mack chemical. One day in the Chemical Plant Zone, an accident occured that flooded this scientist's compartment with the deadly toxin. Whilst the scientist's body had dissolved into the chemicals, his mind remained intact, possessing the Mega Mack itself. With a new body made of liquid death, the scientist became known as Megatox.
In Issue 10, Sonic the Hedgehog raided the Chemical Plant Zone in order to rescue Robotnik's slaves. They tried to escape through a passage that led into a large containment chamber filled with Mega Mack, where Megatox was lying in wait. With his poisonous liquid body, Megatox seemed to have the upper hand against Sonic, able to reform himself whenever struck by Sonic's super spin attack and capable of forming multiple bodies to attack from any angle. Sonic was able to defeat him by burrowing through the floor of the chamber and tunneling through to the outside. As he dug his tunnel, the Mega Mack was drained from the chamber and Megatox was sprayed all over the zone. Sonic and the freed slaves didn't wait around to see if Megatox could recover.
As it turned out, however, Megatox survived this encounter and hoped to avenge himself against Sonic. His opportunity would come several years later - after Robotnik's downfall - when the Marxio Brothers took control of the Chemical Plant Zone. The Mega Mack factory was set to explode and the Marxios were unable to stop it as their workers were being frightened off by some unknown creature lurking inside. Sonic and Tails went to investigate and found the area crawling with creatures who seemed to be made of Mega Mack. The creatures demanded revenge against Sonic, but Sonic didn't remember who they were initially. The creatures then merged together, and Sonic recognised the monster as Megatox (although he actually called him "Mister Tox" at first). Megatox revealed to Sonic that he had learned how to absorb the electricity from the factory and planned to use this power to blow it up. Doing so would allow Megatox to escape and flood the Emerald Hill Zone with Mega Mack, killing everything and leaving the zone uninhabitable for a hundred years. But before he would destroy Emerald Hill, Megatox wanted to kill Sonic. With the electricity he had absorbed, Megatox could make himself solid enough to strike at Sonic as well as being able to poison him by simple touch. Just as he was about to land a killing blow, Sonic struck Megatox square in the face before the creature could revert back to liquid form. The impact knocked Megatox unconcious, ending his threat. The Marxios were then able to power down the factory and clean up the Mega Mack. What became of Megatox after that is unknown.
Being made of Mega Mack, Megatox is completely fluidic and cannot be harmed by most physical attacks. With his toxic form, he can also poison enemies upon touching them. Somehow he can charge himself up with electricity as well, which allows him to become solid enough to perform physical attacks. He can shift between solid to liquid at will, though while solid he can also be attacked.