The people who live on the planet that would become Lexor were once a highly intelligent society based on super science. However, the society regressed in barbarism. The reason were multicolored crystals called Rainbow Crystals, which dull electroencephalic action, lowering intelligence. The great machines that the Lexorians had perfected fell into disrepair as the ages passed.
When Lex Luthor challenged Superman to a fight without superpowers, Superman took the challenge, fearing that people would lose faith in him if it seemed he bullied Lex. Superman agreed to build a rocket to take Lex and he to a planet with a red sun, specifically Red Star X-156-99F. They found the planet that would become known as Lexor When they arrived, Superman gave Lex "gravity shoes", so that Lex would be capable of moving in the super gravity of the planet. Lex took off after this, leaving a powerless Superman to suffer in the desert of the world.
Superman, delusional, wandered into a city that had fallen into disrepair. Luthor stumbled upon people near the city having their crops being assaulted by massive birds. Luthor, not wanting the people to come to harm, drove the crows off with a water pump. The people, noticing Lex's ingenuity, asked him if he could find water for them. He agrees, and has several enormous digging robots built. He finds none, but can not bring himself to tell them. The Lexorians find Superman before he has to, however.
Superman and Lex have their battle in the Lexorian coliseum, utilizing ancient weaponry. Lex throws the fight, however, knowing Superman can throw chunks of ice from other planets to Lexor and dig enormous canals with his heat vision to save the people. Superman agrees. The Lexorians, thinking it was Lex that did this, erect a large statue of Lex with a plague reading "In honor of Lex Luthor for His Many Scientific Feats erected by the people of Lexor". In addition, they name their planet after him.
Later, when Lex and Brainiac are attempting to gather the materials to make a serum to deprive Superman of his powers, they visit Lexor. The radioactive material they need is in the machines that the Lexorians use to live. Lex tells Brainiac to seek out the material on another planet.
Lex later returns to Lexor. When Superman pursues him there, Lex uses a machine to temporarily gain superpowers. Under the guise of the Defender, Lex defeats Superman. However, Superman is able to tell him about the Rainbow Crystals and their destructive effects. Lex allows Superman to destroy the crystals.
Lex later returns to Lexor, and marries his girlfriend Ardora. At some point, they have a son, named Lex Luthor Jr. Years later, Lexor was destroyed when Lex, now clad in his new war suit, was fighting Superman in the atmosphere of Lexor. Lex fired a blast at Superman, but the blast ricocheted off of Superman and into the Neutrarod, a device Lex had invented to quell the dangerous seismic activity Lexor experienced. The planet, now unstable due to reacting elements in the core, exploded, killing everyone save Superman and Lex.
A large piece of the destroyed planet of Lexor made its way to Earth. It was used to create Lex Luthor's island / hideout. Originally named L-Island but later changed to Black Island. From here Lex, Wanda Nordo and the Statler Brothers carried out their plans to destroy Superman.
The first mention of Lexor in the post-Crisis universe was when Lex, having gotten superpowers through the Everyman Project, is battling John Henry Irons. Lex quips that when he takes over the Earth, he may rename it Lexor.