Arnold was the mutant child of Harold Sutton and his wife, two people exposed to high-levels of radiation. His superior intelligence was apparent by the time he was one year old. By nine, he was a scientist working for the military, but physically hideous due to his oversized brain and seen as a freak. When he was ten, he refused to work unless he could have his own island, which he got. Angry at how people used him for his mind, but avoided him as a person, he decided to destroy the world. He built a special rocket designed to be fired into the sun, causing it to go supernova, but he tricked everyone into thinking it was going to be a regular probe.
The Avengers, visiting from Earth-616, were forewarned of the event by having traveled a week into the future. They convinced Nighthawk to help them stop the launch. Then the Avengers and Squadron Supreme went to Brain-Child's lair and attacked him. He countered with robots and science stuff, but the Avengers won anyway. They took away his memories and super intelligence, by way of Doctor Spectrum's Power Prism, transforming him into a normal boy. The Squadron Supreme promised he'd have a normal life.
- Arnold's history parallels that of Franklin Richards of Earth-616, who himself would become Psi-Lord as he grew older. They both gained their mighty mental faculties from radiation, which their parents had absorbed before they were born. In fact, Arnold's father Harold and his mother were themselves physically identical to Reed and Sue Richards, although neither possessed superpowers nor was Harold a notable scientist on his world. However, Arnold's own genius and mental manipulating abilities seemed to compensate for this imbalance between the worlds.