A graduate of West Winds-Wauwatosa High School, where he was staff editorial cartoonist for the school newspaper, Duffy attended the University of Wisconsin, both in Milwaukee and Oshkosh, majoring in political science and history. He produced editorial cartoons for the UWM Post, while continuing his studies at the Milwaukee Institute of Art and Design.
Duffy's work has won many awards, including: First place, World Hunger Media Award (1985 and 1986), Second place, Editorial Cartooning, Best of Gannett Contest (1986, 1995 and 1996), Finalist, Fischetti Editorial Cartoon Competition (1986), Third Place, Editorial Cartooning, Best of Gannett contest (1987, 1989, 1992, 1993 and 1994).
Two collections of his cartoons have been published, A Decade of Duffy (1993) and More of Duffy (1995).
Duffy and his wife have four children.