Henwood was born to Harold and Victorine Henwood in Teaneck, New Jersey and grew up in Westwood, NJ. As a youth Henwood was acquainted with Marxism, but he briefly self-identified with conservatism towards the end of high school. According to Henwood: "Sometime late in high school, I fell under the spell of Milton Friedman and Bill Buckley, and about the first thing I did when I got to college was join the Party of the Right (POR). I got tired of all the pompous rituals, and political sanity returned, bringing me back to the left from which I’d started".