Sara Pichelli, Duncan Fegredo, Kate Brown, Emma Vieceli and Barry Kitson have also been added to the line-up, reports Newsarama.
Snyder is currently writing the bestselling Batman and acclaimed Swamp Thing titles as part of DC Comics' New 52.
Cloonan and long-time collaborator Brian Wood are working on a new Conan series for Dark Horse Comics.
Pichelli is the artist of the acclaimed Miles Morales-starring Ultimate Spider-Man, and is working on the forthcoming Spider-Men project.
Fegredo is best known for his collaborations with Mike Mignola on Hellboy.
Mark Millar's second annual convention will be held at the London Business Design Centre in Islington on May 19 and 20.