Stewart got his start in publishing in science fiction fandom. He published The EC Fan Bulletin, the first EC fanzine in 1953; co-edited the Hugo Award-winning science fiction fanzine Xero (1960-1963), and is credited with coining the term "underground comics" during a panel discussion at a comics convention in 1966. '' He teamed with EC Comics publisher Bill Gaines to choose stories for The EC Horror Library of the 1950s (1971), and later worked closely with Mad's cartoonists while editing the Mad Style Guide (1994) and Gibson's line of Mad greeting cards (1995). In 1980, he became the regular film columnist for Heavy Metal.
Stewart scripted for animation (Kissyfur) and created the short film, The Year the Universe Lost the Pennant (1961). He wrote comics for several publishers (Byron Preiss, Marvel, Warren, Charlton, Heavy Metal); succeeded Vaughn Bode as editor of Gothic Blimp Works, the comix tabloid published by the East Village Other; edited and designed magazines (Castle of Frankenstein, Flashback); devised humor products for Topps; and introduced DC Comics to the world of trading cards as editor of DC's first card series (Cosmic Cards, Cosmic Teams). He taught several classes in comics at the School of Visual Arts. His readings of fantasy stories aired on Pacifica Radio's Midnight Chimes, and he has contributed to numerous newspapers (The Real Paper), magazines (The Realist, Galaxy Science Fiction) and books (Bare Bones).