Changing his name to James Warren, he, his wife, Felice, and their six-week-old son moved to Hollywood, where over the course of several years he appeared in more than 30 movies.

His first picture at RKO, filmed in the spring of 1945, was Ding Dong Williams, a low-budget musical comedy with a Hollywood theme.

RKO had been making Zane Grey westerns with Robert Mitchum in the leading roles, and with Mitchum now advancing to dramatic features, RKO producer Herman Schlom (who had made Ding Dong Williams) remembered how well James Warren photographed in western gear.

At one of these shows, Katharine Hepburn purchased seven of Warren's large watercolor paintings which she kept in her private collection.

Warren specialized in whimsical interpretations of the Hawaiian Tutus (Grandmothers) and Kikis (Grandchildren) as well as many local images.