He began doing such commercial art as billboards and record covers, ghosting for some DC Comics artists, and a job for "Classics Illustrated" comics. Former EC Comics artist Jack Kamen by this time was the art director of Harwyn Publishing's 12-volume, 1958 "Harwyn Picture Encyclopedia" for children, and had Sinnott join a roster of contributors that included such notable EC artists as Reed Crandall, Bill Elder, George Evans, Angelo Torres and Wally Wood. Sinnott also began a long association with publisher George Pflaum's "Treasure Chest", a Catholic-oriented comic book distributed in parochial schools. With Bob Wischmeyer, a "Treasure Chest" writer-editor, Sinnott collaborated on an unsold college-athlete comic strip "Johnny Hawk, All American".