Jax-Ur was once a scientist who worked at the Kryptonain Space Center along with his colleague Jor-El. While there, he developed a nuclear missile and decided to test it against a giant meteor in space. If this test proved successful, Jax-Ur would then commence the build-up of a massive, privately-held nuclear arsenal with which he would overthrow the Kryptonian government, and place the entire planet under his dominion. His missile collided with a spaceship piloted by Jor-El, altering its trajectory. Instead of striking the meteor, it instead struck the Kryptonian moon of Wegthor, destroying the moon and its lunar colony. Although the incident was an accident, Jax-Ur was placed on trial for the deaths of over 500 colonists. He became the first criminal in Kryptonian history to be imprisoned inside the dimension known as the Phantom Zone. Time passed and more and more criminals were exiled into the zone, at which point, Jax-Ur met the military leader known as General Zod. Aligning himself with Zod, he committed his amoral, scientific genius towards avenging himself against Jor-El. Fate would deny him his vengeance however, for as the criminals remained in incorporeal form in the Phantom Zone, those left on Krypton died horribly when the planet exploded due to geological instability.
Years passed, but Jax-Ur and the others learned that the son of Jor-El, Kal-El, had survived Krypton's destruction and were now living on Earth. Jax-Ur eventually found a means of escaping the Phantom Zone and journeyed to Earth. Here, he discovered that under a yellow sun, he developed extraordinary super-powers. By this point in time, Kal-El was a teenager and used his similar super abilities to fight crime as Superboy. Jax-Ur found Superboy's adoptive father Jonathan Kent and imprisoned him, leaving him free to take his place. Masquerading as Kent, he told Superboy that he would now assume the mantle of "super-hero" and began flying around town wearing a costume similar to Superboy's. Jax-Ur revealed himself however when he inadvertently signed his real name on an invoice at the Kent General Store. Superboy discovered Jax-Ur's true identity and defeated him, freeing his father in the process. Jax-Ur was returned to the Phantom Zone.
Jax-Ur was a Kryptonian scientist, who assisted General Zod in his failed coup of Krypton, and was sentenced to 300 years in the Phantom Zone. With the rest of his confederates, he was accidentally released from the Phantom Zone and assisted Zod in his attempt to defeat Superman and conquer Earth.