Gary Hallgren grew up in the Pacific Northwest U.S. and studied painting and design at Western Washington State College. He became a sign painter, specializing in Art Nouveau style and cartoon art. Gary Hallgren became involved with the underground comix scene around 1970. He teamed up with Dan O' Neill, Bobby London, Ted Richards and Shary Flenniken to start Air Pirates Funnies, a comix series that parodied Disney characters (especially Mickey Mouse, who could be seen having sex with Minney, getting high and smuggling drugs). Disney started a process against the artists for copyright infringement, but the case was finally settled out of court.
Gary Hallgren's work appeared in numerous magazines, such as National Lampoon, Chic, Screw, Rolling Stone, Howard the Duck, Crazy, Hustler and Provincetown. He moved to New York, where he worked as a commerical artist for major magazines and newspapers.