Before becoming an editor and writer, Harrison started in the science fiction field as an illustrator, notably with EC Comics' two science fiction comic book series, "Weird Fantasy" and "Weird Science". In these and other comic book stories, he most often worked with Wally Wood. Wood usually inked over Harrison's layouts, and the two freelanced for several publishers and genres, including westerns and horror comics. He and Wood split up their partnership in 1950 and went their separate ways. Harrison used house pen names such as Wade Kaempfert and Philip St. John to edit magazines and published other fiction under the pen names Felix Boyd and Hank Dempsey (see Personal Life below). Harrison ghostwrote "Vendetta for the Saint", one of the long-running series of novels featuring Leslie Charteris' character The Saint. Harrison also wrote for syndicated comic strips, writing several stories for the character Rick Random.