The Superior writer will end his ongoing work on the publisher's comics and dedicate his time to his creator-owned projects.
"Well, I just finished the dialogue on my last ever script for Marvel," Millar wrote on Twitter. "It was a lot of fun, but I'm full-time on Millarworld as of today.
"Huge thanks to all the guys I worked with on Ultimates 1&2, Civil War, Old Man Logan, Enemy of the State, Ultimate X-Men, 1985, etc, etc. It's been an amazing experience and I'm stealing all the best ones to work with my on creator-owned anyway."
Millar continued: "In the next 12 months - Kick-Ass 2, Nemesis 2, Supercrooks, Superior, the Dave Gibbons project, the Frank Quitely project and Hit-Girl!
"I'm going to miss the lads, the best editorial team in the biz, but there comes a time when every boy needs to start his OWN media empire!"
Millar was a key creator for Marvel, heading its major 'Civil War' event, before reducing his involvement following the success of his popular Kick-Ass miniseries, published through the publisher's creator-owned imprint Icon.
Many of his forthcoming projects were announced at Kapow! Comic Con, which Millar himself organised in April.