An alumnus of Hood College and a graduate of Goddard College, Gallaher went on to write for Marvel Comics, the New York City Police Department, Moonstone Books, the CBLDF, Harris Publications and McGraw-Hill.
His works include Yours Truly Johnny Dollar, Box 13, Annex, Moonstone Monsters, High Moon, Darkstar and the Winter Guard, More Fund Comics, Vampirella, and Vampire: The Masquerade. Gallaher's work is typically characterized as being heavily-researched with an emphasis on staccato dialogue and destructive conclusions.
In 2008, Gallaher was named as a 'Breakout Creator' by Comic Foundry magazine. Gallaher was featured in the June 2009 issue of Penthouse magazine. In October 2009, Gallaher earned a Harvey Award for his work on the series High Moon.