First appearance = Wow Comics #1 (Winter 1940-41) from Fawcett Comics
In Wow Comics #4, Mr. Scarlet would get a kid sidekick: Pinky the Whiz Kid (who was Mr. Scarlet's son Pinky).
First appearance in a new story from DC Comics = Justice League of America #135-137 (October 1976 - December 1976)