The Star Wars actor will not be "starring" in the proposed movie adaptation of the comic, but will have a "pivotal" role.
"Can I just stress that Mark isn't starring in Secret Service? He does, however, have a pivotal role in the story," the Kick-Ass creator wrote on Twitter.
The Secret Service, to be published under Marvel Comics' Icon imprint, is Millar's upcoming collaboration with artist Dave Gibbons. X-Men: First Class helmer Matthew Vaughn is on board to turn the series into a movie in the future.
Millar went on to say that he has held talks with Hamill about the project, but did not reveal specific details of the actor's involvement. "I still can't believe I phoned Mark Hamill last night," he added. "I was drawing this dude when I was 7. Still slightly stunned this morning."
The Secret Service #1 is released in April.