While Kaluu plotted further conquests and the people of Kamar-Taj fell into decadence, the paralyzed youth, whom Kaluu had placed in the public square as an object of ridicule, mentally called upon mystical forces to defeat Kaluu. However, the youth was not yet a full master of the mystic arts, and, although hr did not intend it, the mystic forces he summoned creates a pestilence that wiped out virtually the entire population of Kamar-Taj. Kaluu fled to another dimension. (He was defeated by, Dr. Strange centuries later.) With Kaluu gone, the youth was freed from his paralysis. Ironically, Kaluu's spell had protected the youth from the full effect of the mystical pestilence: the youth was no longer immortal, but he would now age at a very slow rate, so that he would nor reach the end of his life span for five centuries.

Because of what he had witnessed in Kamar-Taj, the youth left the village in order to devote his life to combating other evil sorcerers. To this end, the youth sought out an ancient order of sorcerers whose elder members were known as Ancient Ones, and which was dedicated to protecting Earth's people from black magic. The youth continued his mystical studies with them, and rapidly became far more skilled in sorcery than his colleagues. His prowess was so great that he was the first Earth mortal to meet with Eternity, the sentient embodiment of the life forces of the universe, who presented him with the amulet of Agamotto, the Earth dimension's very first sorcerer supreme. The amulet enabled its possessor to protect his or her dimension from the sinister extra-dimensional entity known as Dormammu, and using it, the man who became known as the Ancient One kept Dormammu out of the Earth dimension for nearly five centuries, although he never succeeded in entirely defeating Dormammu. Through this and other feats, the man who became known as the Ancient One became recognized as the sorcerer supreme of the Earth dimension. The Ancient One further increased his knowledge even within the last century by traveling back in time to ancient Babylonia, defeating the gryphon whom the priests of the Babylonian god Marduk had set guard over the Book of the Vishanti, and thus obtained the book, the greatest known source of "white" magical knowledge.

Within the last several decades, the Ancient One, who had outlived all the other members of his order, was approached by Baron Karl Mordo of Transylvania, who wished to become his pupil. The Ancient One was nearing the end of his long life and realized he needed a successor. Although he recognized Mordo's evil ambitions, he also sensed Mordo's talent for sorcery, and he decided that he could at best convert Mordo to the service of good and at worst keep him in check.

Sometime later, a gifted but somewhat amoral surgeon, Stephen Strange, sought out the Ancient One to cure his hands, which had been injured in an automobile accident in a way that prevented him from holding his surgeon's tools with the complete steadiness that was necessary to perform surgery. While staying at the Ancient One's palace in the Himalayas, Strange learned that Mordo was plotting against the Ancient One. Mordo placed a spell upon Strange which prevented him from warning the Ancient One about Mordo, but the Ancient One freed Strange of the spell and assured him that he was aware of Mordo's evil nature. Having witnessed the power of good and evil magic in action, Strange found a new, higher purpose in life, and became the Ancient One's disciple. Mordo left the Ancient One's tutelage sometime afterwards to embark on his career of evil. Dr. Strange, on the other hand, remained with the Ancient One for years, proved to be highly talented in the mystic arts, and eventually became the Ancient One's successor as sorcerer supreme.

Some years after Doctor Strange had left the Ancient One's palace to return to New York City, the Ancient One and Dr. Strange fought the powerful demon Zom at Stonehenge in Britain. Knowing that he could not long withstand Zom's power at his advanced age, the Ancient One made it appear as if Zom had killed him by fusing his body with one of the Stonehenge monoliths. As he seemed to die, the Ancient One claimed to transfer his magical abilities to Dr. Strange; instead, the Ancient One actually activated potential for further magical ability in Dr. Strange that had until then remained latent. Zom was imprisoned by the Living tribunal, and the Ancient One, still in possession of his powers, was reunited with Dr. Strange some days later. The Ancient One then entered full retirement, allowing Dr. Strange to assume his position as sorcerer supreme.

Years later the Ancient One was taken prisoner by the so-called Living Buddha and minor demons called the Shadowmen, all of whom served the extradimensional demon Shuma-Gorath. They intended to use his waning power to open the way to earth for the demon. The Ancient One retained enough consciousness to mystically make his own mind the only conduity through which the demon could travel. Knowing that he had long outlived his natural life span, the Ancient One prompted his disciple Dr. Strange to "kill" him by destroying that portion of his mind which constitutes his ego or sense of self. With the Ancient One's brain shut down, Shuma-Gorath was trapped in the dead brain. With his body dead, the Ancient One's astral form was set free and the aged mage willed it to become one with the physical universe.

The Ancient One has harnessed his still potent magical energy to reappear on the Earthly plane on two separate occasions. The first time he manifested himself as an avatar(or discrete aspect) of Eternity. The second time he regained corporeal from against his will when the evil creators caused the universe to reject him. This drove the Ancient One mad for a time, but he has since regained his sanity, again given up corporeal form, and resumed his mystic unity with the universe.

As the Sorcerer Supreme of Earth for several centuries, the Ancient One possessed a greater knowledge of and proficiency in the mystic arts than any other human being of his era. Through centuries of study, discipline, and enlightenment, he developed his natural aptitude for sorcery to its theoretical peak.

The Ancient One’s personal powers included highly advanced mental and spiritual disciplines. He was a master of astral projection, enabling him to separate his astral form—the metaphysical embodiment of his consciousness and soul—from his physical body and travel freely across vast distances, unconstrained by physical laws. In this state, he retained full awareness and could interact with other astral entities, though his physical form remained in a vulnerable, trance-like state. At the height of his abilities, he could maintain this separation for extended periods without degradation.

He also possessed formidable telepathic and psionic capabilities. The Ancient One could communicate mentally across planetary distances, provided he could focus upon the intended recipient. He was capable of simultaneous communication with multiple minds and could exert hypnotic or mesmerist influence over others, both at close range and remotely.

By tapping into ambient mystical energies, the Ancient One could produce a wide variety of effects. These included the projection of powerful bolts of magical force, the generation of highly resilient protective shields, the manipulation and transformation of matter, and the creation of illusions or light-based phenomena. He could also teleport himself across great distances or between dimensions, although such feats required considerable energy and were used sparingly in his later years due to the strain imposed upon his aged physical form.

The Ancient One was extensively trained in the use of spells and incantations that invoked extradimensional sources of power. By calling upon entities such as the Vishanti, he could channel vast energies without depleting his own reserves. Among the many manifestations of such power were the Flames of the Faltine, the Hoary Hosts of Hoggoth, and the Shades of the Seraphim. These invocations allowed him to achieve effects far beyond the limitations of personal or ambient energy manipulation.

Despite his immense power, the Ancient One’s physical body became increasingly frail with age. In his final years, the exertion required for major magical feats placed severe strain upon him, limiting his direct involvement in prolonged mystical combat.

Following his physical death, the Ancient One transcended mortal limitations and achieved a state of unity with the cosmos itself. In this form, his consciousness became one with the fabric of the universe, and he has since demonstrated the ability to manifest as a spiritual or cosmic presence. In certain instances, he has acted as an aspect or agent of Eternity, suggesting access to vastly greater levels of power than he possessed in life.