Growing up outside of Bellingham, Washington, Hallgren studied painting and design at Western Washington State College. He joined the underground comix scene sometime around 1970, publishing his first narrative story in Northwest Passage, a local underground newspaper. In early 1971 cartoonist Dan O'Neill invited Hallgren — then based in Seattle — and some other artists to San Francisco to form the Air Pirates collective. The Air Pirates lived together in a warehouse on Harrison Street in San Francisco. They teamed up to produce two issues of Air Pirates Funnies. Hallgren's work in Air Pirates Funnies, Pollyanna Pals, imitated Cliff Sterrett's old-time comic stripPolly and Her Pals. Air Pirates Funnies contained parody versions of Mickey Mouse, which led to a highly publicized lawsuit from The Walt Disney Company. Hallgren also did the cover artwork and stories for a follow-up comic, The Tortoise and the Hare . The initial court decision, delivered on July 7, 1972, went against the Air Pirates, and O'Neill's lawyers appealed to the United States Court of Appeals for the Ninth Circuit. O'Neill suggested the other Pirates settle, and leave him to defend the case alone. Hallgren and the publisher, Ron Turner, settled with Disney. Other underground comix to which Hallgren contributed included Comix Book, San Francisco Comic Book, and Manhunt #2. From about 1986 to 1988, Sherman was part of Upstart Associates, a shared studio space on West 29th Street in New York City. Founded by Howard Chaykin, Walt Simonson, Val Mayerik, and Jim Starlin; the membership of the studio changed over time, and at the point Hallgren joined, Upstart consisted of Simonson and James Sherman. Hallgren left the studio when he bought a house in Long Island. He illustrated the four-issue limited seriesMort the Dead Teenager, written by Larry Hama and published by Marvel Comics in 1993–1994. In 1994, Hallgren was one of a number of cartoonists who applied to take over the long-time syndicatednewspaper stripNancy. Hallgren illustrated Mehmet Oz's book, YOU: The Owner’s Manual: An Insider’s Guide to the Body That Will Make You Healthier and Younger, published in May 2005 by William Morrow Paperbacks. As of May 2006 Hallgren lived in Granby, Massachusetts and had his studio in Holyoke.
