Billions of years ago, the technologically-advanced extraterrestrial race known as the Watchers decided it was their duty to help the universe's less advanced races. In their first such experiment, proposed by Ikor, a delegation of four brought atomic energy knowledge to the planet Prosilicus. The Prosilicans accepted the gift, but used it to develop nuclear weapons and engaged in an auto-genocidal war. Ashamed, the Watchers vowed never again to interfere in the affairs of other races. Ikor drafted a code of ethics based upon strict noninterference and passive observation that was so strict that if another being were dying at a Watcher's feet, the Watcher would offer no aid. With their new code established, the Watchers evacuated their homeworld and relocated throughout the galaxies, each Watcher choosing a solar system where they could observe and mentally record the lives of other races for the sake of eventually sharing it with their fellow Watchers. The Watchers would then sporadically reassemble to pool their knowledge and share what they had witnessed.
Ikor's son, Uatu, chose Earth's solar system as his new home and was based in the "Blue Area" of Earth's moon, an abandoned settlement built by the alien Kree and Cotati, from which he observed Earth for billions of years. At first, Uatu adhered to his race's vow of non-interference: he did not physically intervene when the Gunan known as Xakku the Sun-Stealer threatened to transport Earth's sun to his own solar system (although he did trick the alien into expending his oxygen tanks before he could accomplish his mission) and even spurned the affections of the extraterrestrial Queen Kalthea, who offered Uatu her hand in marriage after his appearance caused the barbarians threatening her kingdom to retreat; but over the centuries, Uatu grew so fond of the lifeforms he observed that he occasionally violated his vow of non-interference when Earth's existence was threatened. Uatu's first recorded encounter with an Earthling occurred in the year 1602 A.D. when a possible-future Captain America was inadvertently sent back in time to that era, causing ripples in the timestream that threatened the very fabric of reality. Uatu revealed the nature of the problem to an Elizabethan-era version of Dr. Stephen Strange, who helped send the temporally-displaced Captain America back through a time-portal, correcting the timeline. Uatu's superior, however, allowed the divergent timeline (Earth-311) to persist in a pocket dimension. Decades ago, Uatu assisted the Destroyer (Roger Aubrey), Iron Cross (Helmut Gruler), Nathaniel Richards, and Howard Stark on a mission to save mankind.
In modern times, Uatu met the Fantastic Four when they visited the Moon to investigate the Blue Area. The criminal mastermind Red Ghost and his Super-Apes had also landed there and soon attacked the Thing. Uatu revealed his presence to the Earthlings and ended the fight, decreeing that the Fantastic Four would battle Red Ghost and his simian cohorts. Uatu was impressed with the FF's victory and gave them his blessings, allowing them to retain their memories of his existence. Uatu became fond of the Fantastic Four and began to warn them of impending dangers, such as the Molecule Man, who gained matter-manipulation powers in a lab accident. After the Molecule Man's defeat, Uatu transported the villain to an other-dimensional world where time passed at an accelerated rate and reinforced the self-imposed mental blocks which limited Molecule Man's ability to use his powers.
Soon after, at the wedding of Mr. Fantastic and Invisible Woman, Uatu helped Mr. Fantastic defeat the horde of super-villains who threatened the ceremony by transporting him to the Watcher's home and not dis-allowing him to pick a weapon to send the villains back to before they were controlled, and erase their memories. Afterwards, Uatu allowed Reed and Sue to honeymoon in the Blue Area's city, providing an opulent banquet hall and assuring the newlyweds that they would be the only beings in the galaxy that he would not be watching that night. Later, when Galactus first menaced Earth, Uatu tried to hide the planet from the world-devourer, debated him as to why the Earth should be spared, and finally helped the Human Torch obtain the only weapon capable of destroying the cosmic menace -- the Ultimate Nullifier. Uatu also studied alternate realities, which he observed through a portal given to him by the Time-Keepers, a group of beings who existed at the end of time itself.
Eventually, Uatu disgraced himself completely by physically opposing the Kree soldier Captain Mar-Vell after allying himself with the Lunatic Legion, a Kree faction opposed to Mar-Vell who were led by Zarek, Fer-Porr, and Sro-Himm and based in the Moon's Blue Area. Placed on trial by his own people for his numerous violations of their ethics code, Uatu explained that he had come to identify with the action and emotion that he observed on Earth, and thus yearned to become involved. At the conclusion of his trial, Uatu was released from custody on his own recognizance after promising never to interfere in the affairs of other races again.
Later, Uatu was captured by intergalactic gambler A'Sai via a weapon from one of Galactus' former worldships. Uatu allowed himself to be coerced into a football wager-- Uatu's technology vs. A'Sai's weapon--but when Spider-Man led Uatu's team to victory, Uatu destroyed the weapon and transformed A'Sai into a casino playing card as punishment for his intent to renege. Uatu also saved the life of Bette Chekhov, granddaughter of Anton, one of May Parker's former boarders. When the criminal Living Laser tried to escape Quasar by flying into Uatu's alternate worlds portal and diverged into separate beings in seven different alternate realities, Uatu enlisted Quasar to recapture the Living Laser. Based on the Watcher Otmu's theory that even observation affected others, the "Oblivion Plague" caused many Watchers to end their existences; though perpetuated by Maelstrom, the Plague was ended when Quasar convinced the Watchers that they must continue to observe so they could pass on their accumulated recordings to the next universe.
Present at Uatu's earlier trial was his nephew Aron, a relatively young Watcher mentored by his uncle who was not yet assigned a solar system to observe. Fascinated by Earth, Aron eventually broke the Watcher's code by manipulating the Fantastic Four and Inhumans into a fight. Now a "rogue" Watcher, Aron grew more malevolent; after cloning the Fantastic Four and trying to create a pocket world within the mind of Alicia Masters, Aron was ambushed by Dr. Doom, who siphoned-off Aron's power into a containment device. Uatu cared for Aron as his nephew recovered; but before Aron could be tried for his crimes, the Watchers became preoccupied with the threat posed by the giant cosmic entities known as the Celestials. Aron seized this opportunity to masquerade as his uncle Uatu and manipulate the Fantastic Four into helping him recover the energy stolen from him by Doom. Completing the first phase of his plan, Aron plotted to swallow the entire universe into a black hole and then create a new universe for himself to manipulate. Opposed by the Fantastic Four and Fantastic Force, Aron pitted legions of their past enemies against them, but Ant-Man foiled his scheme by destroying the device needed to create the black hole. In the end, Uatu disciplined his nephew by dissipating his energy and using it to revive the leader of the Watchers known as the One. Uatu was stripped of his title and exiled by the Watchers for these controversial actions.
Despite his expulsion, Uatu continued to observe affairs on Earth, compelled by the lives and struggles of its inhabitants. Eventually reinstated into the Watchers, Uatu witnessed the temporary demise of Galactus, who was destroyed by an intergalactic alliance of heroes led by the Silver Surfer near the Shi'ar homeworld. Soon after, Uatu was viciously attacked by Abraxas, a cosmic entity threatening all of reality, but Abraxas was defeated by the Fantastic Four. Uatu congratulated the heroes, informing them that humanity had passed another test.
Some time ago, Uatu and two of his fellow Watchers, Zoma and Qyre, appeared before She-Hulk, who was then serving with the Magistrati as a presiding judge in the Living Tribunal's Cosmic Claims Court. Qyre had discovered a reclusive race who had kept their existence secret since time immemorial. The reclusive race sued the Watchers in the Cosmic Claims Court, and She-Hulk ruled that Qyre's mouth be permanently sealed so that he could not share his knowledge of the plaintiffs with his fellow Watchers.
Most recently, Uatu the Watcher appeared before the costumed heroes. They were at a meeting to discuss what to do after the Civil War|incident at Stamford. As Watchers only show up personally to monitor key events in history, his presence did not bode well for the costumed heroes.