The handsome Hull made his screen debut in 1934 for Educational Pictures, a short-subject studio. He co-starred opposite singer Sylvia Froos in the "Young Romance" series of musical comedies filmed in New York; Hull often joined Froos in song. In 1935 Hull was signed to a contract by Warner Bros., and spent the next few years playing leading men both in dramas and musicals. His best appearance of this period came in The Walking Dead 1936), a horror movie starring Boris Karloff and directed by Michael Curtiz. Some of Hull's early appearances have him billed as "J. Warren Hull."
When his Warner contract expired, Hull had no trouble finding work at other studios. He teamed with Patricia Ellis, one of his leading ladies at Warners, for the 1937 Republic Pictures musical Rhythm in the Clouds. He also played romantic leads in a string of features for Monogram Pictures.
In 1938 Columbia Pictures terminated its association with the Weiss Brothers, independent producers who had been making adventure serials for Columbia release, and decided to make its own cliffhangers. Warren Hull was signed for Columbia's first (and probably best) serial production, The Spider's Web (1938), based on a popular magazine character. Hull played three parts: criminologist Richard Wentworth, his masked-and-caped alter ego The Spider, and in a second masquerade, lowlife mobster Blinky McQuade. The personable Hull brought a breezy sense of humor to his serial roles; he is probably the only serial hero who ever laughs on screen! Hull kept audiences following the Spider's thrilling exploits, making The Spider's Web the most popular and profitable serial of the year, outstripping such worthy cliffhangers as Buck Rogers and Dick Tracy Returns by a wide margin, according to a tally published in the Motion Picture Herald.
Pleased with Hull's performance, Columbia cast him as Mandrake the Magician in its 1939 serial. Universal Pictures starred the now-established serial hero in The Green Hornet Strikes Again! (1941) and Columbia put him back in the mask and cloak for The Spider Returns (1941).
In the mid-1940s Hull returned to radio announcing, appearing with frequency on such programs as The Hit Parade and Vox Pop. Hull was also the emcee of Spin to Win, only the second game show created by the team of Mark Goodson and Bill Todman. During the next two decades he hosted TV-hit programs as Top Dollar, Who in the World, and Beat the Odds; he is best remembered as the host of Strike It Rich.