Thomas "Black Tom" Cassidy was the heir to his family fortune and estate of Cassidy Keep in Ireland. However, he wagered both the fortune and estate on a throw of dice, and thereby lost them to his younger cousin, Sean Cassidy. Black Tom and Sean Cassidy were rivals for the love of Maeve Rourke, and Sean eventually prevailed and married her.

While still a young man, Black Tom Cassidy turned to crime as a way of seeking thrills. Discovering how much he like it, Black Tom decided to make crime his profession. At first he masked his criminal activities under the guise of being a soldier of fortune. As such, he roamed the world offering his services as a gun for hire, while pursuing his criminal activities as well. While imprisoned in a Third World jail for acting as a mercenary for revolutionary forces that were defeated, Black Tom Cassidy met a fellow mercenary, Cain Marko. Cassidy and Marko struck up a friendship, and Cassidy used his own mutant powers, which he had already discovered, to enable both of them to escape jail.