The Green Mask came in two different guises, both of whom were published by Fox Features.
The Green Mask (i) debuted in Mystery Men Comics #01 (August 1939) and his final appearance was in Mystery Men Comics #31 (February 1942). He also appeared in his own comic 'The Green Mask' 01-09 (1940-1942).
Originally his identity was kept secret from the readers as well, his real identity known only to a newspaperman, who could contact him when needed.
He was eventually revealed to be Michael Shelby, whose father was a crusading Congressman who was gunned down by the 'Grim Circle'. Michael is mortally wounded during the attack and family friend Professor Lascomb rushes to save the young man and subjects him to his untested Vita-Ray machine. The machine subjects him to a super-charge, and when he comes to, Shelby is not only healed but he's a miracle man, with super-strength, invulnerability and the ability to fly.
He vows vengeance on his father's murderers and all crime and decides to take up an identity that will strike terror in the hearts of criminals, 'The Green Mask'. He is assisted by his side-kick, 'Domino, The Miracle Boy' who after getting hurt in an explosion, which was meant for 'The Green Mask' is subjected to the same treatment but doesn't gain all the same powers as he cannot fly. Shelby's girl-friend is Olivia Tracy.
The Green Mask (ii) made his first appearance in Green Mask #10 (August 1944). His last appearance was in Green Mask #17 (November 1946).
In this incarnation, 'The Green Mask' is secretly Johnny Green. He is the son of a previous Green Mask, named Walter Green, who is serving in the army in Europe.
Johnny is a shy bespectacled teenager often taunted as a 'sissy boy' by his high school peers. He transforms into an adult 'Green Mask' whenever he becomes angry and cries out at any display of injustice 'Eee-ow!'. He changes back once he has completed his mission, grows tired and yawns, waking up as Johnny Green again. While some stories had him aware of his alter-ego, others indicated that he either didn't remember anything he did while transformed, or simply saw them as only a dream.
He has a dog named Curly and a much more out-going and courageous girlfriend named Suzie who is unaware of his alter-ego, but is a big fan of 'The Green Mask'.