Little is known of the early life of the crime boss known as "Killer" Maroni prior to his emergence as a major figure in New York's organized crime scene in the early 1940's.  His first clearly documented activity was when one of his agents attacked an informant who was severely injured, but not killed on their way home.  Placed under police protection, the informant was tended by Dr. Charles McNider until a bomb was thrown through the window, killing everyone in the room except  McNider.    The physician was taking to surgery and recovered but was rendered blind by the explosion.  The assailant reported his success back to Maroni, who was exceptionally pleased.

McNider recovered and no longer able to practice medicine, turned to writing "true crime" stories.  Maroni was a fan of such magazines as they tended to glorify racketeers and other figures of organized crime but McNider's pen was much more poisonous.  He painted gangsters as cowards and criminals, unworthy of the respect of decent men.  Maroni was incensed by the lack of positive coverage but declined to openly assault McNider, fearing reprisal and expecting the words of a blind man to carry little weight.  Instead he turned to his protection racket, in which a local candy store owner under Maroni's intimidation had informed the police. Sending men to the shop, the shop owner was gunned down and left for dead on the floor of his store.

The next day, Maroni was enraged to learn that the shopkeeper had survived but was in critical condition, scheduled for surgery at a hospital.  He send his minion to pull the fuses at the hospital, plunging it into darkness so no one could operate.  McNider, now the adventurer known as Dr. Mid-Nite, arrived at the hospital as the lights when out and, able to see in the dark, was able to perform the surgery himself, saving the new informant. When Maroni's henchmen returned with the news of success, he was followed by Dr. Mid-Nite who informed them of their turn of bad luck. Dousing them in the clouds of new "blackout bomb", Md-Nite used the cover of darkness to thrash all of them and bind them over to the police. 

Whether Maroni was convicted and what sentence he received is unknown.  Given he was middle-aged in the 1940's, it is unlikely he is still alive on Earth-Two.  Whether his operation was related to the Maroni/Moroni crime family whose boss disfigured Harvey (Two-Face) Kent the following year (Detective #66) is unclear and whether the Maroni crime organization persisted in any form after his arrest is unknown.