Illustrations by Hallgren have been "commissioned by publications such as The New York Times, Men's Health, The Wall Street Journal, Mad, and Entertainment Weekly, among others.

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.

[2] Air Pirates Funnies contained parody versions of (among other figures) Mickey Mouse, which led to a highly publicized lawsuit from The Walt Disney Company.

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.

[1] Since 2015, Hallgren has been providing art for Hägar the Horrible, a popular newspaper comic strip distributed by King Features Syndicate.