In 1995 Millidge began his Strangehaven series and in the same month published a short Strangehaven story in the comics anthology Negative Burn.

Millidge also writes about comics, editing Portrait of an Extraordinary Gentleman and writing Comic Book Design.

Comics

Comics work includes:

• Strangehaven #1- (script and art, Abiogenesis Press, 1995-) collected as:
• Arcadia (collects #1-6, foreword by Dave Sim, 2001 ISBN 0-946790-04-3)
• Brotherhood (collects #7-12, foreword by Bryan Talbot, 2001 ISBN 0-946790-05-1)
• Conspiracies (collects #13-18, foreword by Dave Gibbons, 2005 ISBN 0-946790-07-8)

• "Strangehaven: Custard Creams" (script and art, in Negative Burn #24, Caliber Comics, 1995)
• "From Hell and Back" (script and art, in Bart Simpson's Treehouse of Horror #9, Bongo Comics, 2003)

Non-fiction

• Portrait of an Extraordinary Gentleman (edited by Gary Spencer Millidge and Smoky Man, 352 pages, Abiogenesis, December 2003, ISBN 0-94679-006-X)
• Comic Book Design: The Essential Guide to Creating Great Comics and Graphic Novels (160 pages, Random House, July 2009, ISBN 0-8230-9796-X)

As well as numerous nominations, including one for the 1997 "Talent Deserving of Wider Recognition" Eisner Award, he won the 1997 "Best Self-Published/Independent Comic" National Comics Award for Strangehaven.

Alan Moore: Portrait of an Extraordinary Gentleman was also nominated for the 2003 Bram Stoker Award for Best Non-Fiction.