Actor James Spader will play Marvel supervillain Ultron in Avengers: Age of Ultron, the May 2015 sequel to The Avengers. Follow this link to see Ultron’s appearances in Marvel comic books.
Ultron made his first Marvel appearance in Avengers No. 54, his true identity hidden as Crimson Cowl. Using Marvel’s digital comic reading tool, you can sample his appearances in Avengers No. 55 (pictured), Avengers No. 201 and Avengers No. 202. You can read more about Ultron’s comic book legacy at Wikipedia (spoilers follow):
The character leads the Masters of Evil against the Avengers, having hypnotized Edwin Jarvis into working for him. In the following issue, #55 (Aug. 1968), the character is identified as Ultron-5, the living automaton, although his origin is still unknown. In Avengers 57 – 58 (Oct-Nov. 1968) in aflashback sequence it is revealed that Ultron is the creator of the “synthezoid” the Vision, whom it tries to use as a weapon to destroy the Avengers.