In his first appearance, Captain Marvel Jr saves an old couple from being run over by a car. The crime-fighter notices the man who ran them over is a safecracker, named Bull. Bull threatens to reform if Captain Marvel Jr. tells the family of his past criminal activities. The young hero gets involved and finds that the couple are living in a poorhouse because of their debts. He helps them out and takes the place of their son.
Junior, along with his sister Mary and the adult superhero Captain Marvel, started appearing in The Marvel Family comic book beginning in 1945. They also began appearing in other Fawcett titles, including Whiz Comics and Master Comics. Junior, clad in a yellow-on-blue costume with a red cape, was drawn by Mac Raboy. His look was a bit more realistic than the cartoony work of Beck and Costanza, which suited the more mature nature of this series.
In a story written by Ed Herron and illustrated by Raboy, the young superhero battles Captain Nazi in Whiz Comics #25 (December 1941). The character made his final appearance in the pages of Master Comics in 1943.
In the 1970s, DC bought the rights to the Marvel Family characters, and reintroduced them with a new title, Shazam!. The retcon also revealed that Junior’s long-lost brother, Kid Eternity, is now his twin. This is because the characters both share an origin that involves their dead grandfathers and rely on magic words to activate their powers.