Bannon began his broadcasting career on local radio station KCKN, then briefly at KMOX in St. Louis. He moved to Los Angeles in 1937, beginning his show Business career in radio as an announcer on The Great Gildersleeve, The Chase and Sanborn Hour, and Stars over Hollywood, among others, with his most prominent acting role being that of Detective Jack Packard in the serial I Love a Mystery. A motion-picture adaptation of the show, with Bannon reprising his radio character, was released by Columbia Pictures in 1945 in hopes of launching a franchise, but only two additional pictures would be produced; he later described the original film as "a weakened product" in his 1975 autobiography.