He claims he is the legendary Norse thunder deity Thor, sent to Earth by his father Odin. Official records say he is Thorlief Golmen, who stole his scientist brother Gunnar Golmen's super-soldier technology after suffering a nervous breakdown. Both accounts agree that he believed he was different from an early age, but only came to believe he was Thor after suffering a nervous breakdown just before turning 30, spending the next 18 months in an asylum. After discovering (or stealing, as the case may be) his powers, Thor began spreading his message of social change through lecture tours and books. He initially refused to join the Ultimates, but did help them subdue the monstrous Hulk after the U.S. President met Thor's demand of doubling the U.S.'s foreign aid budget. Thor began working with the Ultimates thereafter, forming a fast friendship with teammate Tony Stark (Iron Man). He helped the Ultimates in their attempt to recruit Hawk-Owl; though Thor declined fulltime Ultimates duty, he reliably aided the team against serious threats. He helped defeat the extraterrestrial Chitauri invasion force, and neutralized a Chitauri doomsday weapon which would have destroyed the solar system. After failing to prevent Magneto from breaching the S.H.I.E.L.D. Triskelion, Thor battled X-Men members Storm and Colossus, finding the latter's strength matched his own. When Norman Osborn's "Six" attacked the White House, Thor defeated fellow lightning wielder Electro. He also helped the Ultimates and Fantastic Four gather information about the world destroyer Gah Lak Tus.

Thor met (or hallucinated) fellow Asgardian warrior Volstagg the Voluminous, who warned that Thor's evil half-brother Loki had escaped from the "room without doors," and that the Ultimates were pawns of the New World Order. Quitting the Ultimates in protest after they began operating uninvited in foreign countries, Thor was publicly accused by Captain America of leaking team secrets. Thor later intervened to protect protestors from police at an Italian demonstration against the European Defence Initiative's super-soldier program. Persuaded by Gunnar Golmen that Thor was simply a madman using stolen technology, the Ultimates and the E.D.I. hunted down Thor in Norway, attacking him despite his claims that Gunnar was actually Loki in disguise. Thor held his own until Quicksilver removed his belt, depriving him of his powers.

He was then moved to the same holding cell that Bruce Banner had previously occupied. When Loki and the Liberator's attacked, the Ultimates realized that Loki had been deceiving them. The Scarlet Witch then summoned Thor from his cell to finish off Loki. Loki tried again to warp reality to make Thor appear insane, but because the Ultimates' faith in Thor had been restored, his powers didn't work. As a last-ditch effort, Loki summoned the beasts of Asgard to attack the Ultimates. Thor then called on his father, Odin, who sent reinforcements in the form of the Knights of Asgard, to aid the Ultimates in their fight. While the Ultimates fought the monsters, Thor battled Loki, and eventually banished him back to Asgard.

With the battle over, Thor tells the Ultimates that the planet is now under his protection. With the team now knowing that he is truly a God, Thor rejoins the now government-independent team once more.