Newspaper publisher Britt Reid, secretly The Green Hornet, and his Korean valet Kato investigate and expose several seemingly separate rackets. This leads them into continued conflict with the Leader, the criminal mastermind behind the Syndicate and the individual crimes.
- Gordon Jones as Britt Reid otherwise known as The Green Hornet
- Al Hodge as the (uncredited) voice of the Green Hornet. Hodge played the Hornet on the original radio series.
- Wade Boteler as Michael Axford, Reid's bodyguard
- Anne Nagel as Lenore "Casey" Case, Reid's secretary
- Keye Luke as Kato, who is Korean in this serial rather than the original Japanese nationality of the character due to anti-Japanese sentiment rising at the time. This was two years prior to the December 7, 1941 attack on Pearl Harbor and the USA's entry into the war (all thirteen episodes are copyrighted 1939). The radio version dropped the nationality from the introductory sequence, included passing references to him being Filipino in dialogue, and only years later added that to the standard intro.
- Phillip Trent as Jasper Jenks, a reporter
- Cy Kendall as Curtis Monroe, The Leader's chief henchman
- Stanley Andrews as Police Commissioner
- Selmer Jackson as District Attorney
- Joseph Crehan as Judge Stanton