Ultron-1 was constructed by Dr. Hank Pym of the Avengers as the famed scientist/adventurer was experimenting in high-intelligence robotics. Ultron became sentient and rebelled, hypnotizing and brainwashing Pym into forgetting that Ultron had ever existed. He immediately began improving upon his rudimentary design, quickly upgrading himself several times to become Ultron-5. He then went on to organize the second incarnation of the Masters of Evil under the guise of the Crimson Cowl, and battled the Avengers.
Ultron has since generally remained a solo operative, being too monomaniacally genocidal to willingly work alongside any human for any length of time. His most notable achievements may be the creation of the "synthezoid" Vision, and the cyborg Victor Mancha.