By the late 1930s, Delmar was an announcer on such major radio series as "The March of Time" and "Your Hit Parade". He played multiple roles in "The Mercury Theatre on the Air"'s October 1938 radio drama "The War of the Worlds". His main role was that of Captain Lansing, the National Guardsman who collapses in terror when confronted by the Martian invaders, although he also is noted for his address to the "citizens of the nation" as the Secretary of the Interior, in which role he spoke in a stentorian, declamatory style deliberately reminiscent of then-President Franklin Roosevelt. "Cavalcade of America" featured him in their repertory cast, and also was heard as Commissioner Weston on early episodes of "The Shadow".