- Ralph Byrd
- Dr. Jerry Sheehan
- Herbert Rawlinson
- Sir James Blake
- Joan Barclay
- Hope Mason
- Lloyd Hughes
- Dr. Marshall
- Dickie Jones
- Bobby Mason
- Lucille Lund
- Duchess [Chs. 1-6, 14]
- Nick Stuart
- Julot [Chs. 2-4, 9-10, 14]
- Sam Flint
- Chief Insp. Henderson
- Gail Newbury
- Mimi - Policewoman [Chs. 2-4, 9-11]
- Jimmy Aubrey
- The Hag / Baron Polinka
- Theodore Lorch
- Daggett - the Butler
- George DeNormand
- Pedro - Henchman [Chs. 1-7, 14]
- Bob Terry
- Peyton - Henchman
- William Farrel
- Count Basil Segaloff
- Frank Wayne
- Charles - the Footman
- Dick Curtis
- Nicky - Henchman
- Victor Adamson
- Newsboy
- John Bagni
- Cafe Waiter [Chs. 1-3, 10, 14]
- Bruce Bennett
- Adolph - Henchman with eyepatch
- John Elliott
- Landlord and League Representive [Chs. 11-13]
- Henry Hall
- Scotland Yard Detective [Chs. 5, 7, 11-13] / voice of The Scorpion (uncredited)
- John Harron
- Sgt. Dickens [Chs. 6, 8, 11-12]
- Eric Mayne
- Peace Delegate [Chs. 1 & 11]
- George Morrell
- Adm. Blandon [Chs. 1 & 11]
- Francis Walker
- Henchman
