Little was made known about the history of the Ancient One, who never spoke of her past. However, it was revealed that she was Celtic, as well as the latest in a long line of Sorcerers Supreme who were responsible for safeguarding the world against mystical or magical threats. Hence, she was one of the most powerful practitioners of magic on Earth.
After Dr. Stephen Strange gets his hands mangled in an automobile accident, he finds the Ancient One and asks her to heal his hands. The Ancient One, realizing that Doctor Strange is the next in line to become the Sorcerer Supreme, teaches Strange everything she knows about magic, and thus Strange becomes the next Sorcerer Supreme.