Sean was born in his parents' castle, Cassidy Keep, in Ireland. Like most mutants, when arrived at adolescence, he discovered his ability of producing what he calls a sonic scream, which he can use as raw energy or even to fly. Lately, it has been shown that he also can use it like a radar and in more sophisticated applications.

After marrying, he joined Interpol, but while he was in a mission, his wife died, leaving a baby daughter of whose existence Sean didn't know, and she was raised by the evil Black Tom, who desired to use her for his own profits. Therefore, Tom kept her existence secret from her father.

Banshee was forced to join a criminal organization against his will, but got the help of the X-Men to leave them, later joining the group for a while. Then, he met Moira McTaggert, and a deep romance bloomed between them until the Shadow King incident broke them apart.

Later, and in the middle of the Phalanx Alliance issues, he decided to join Emma Frost (the good old White Queen) in her Massachusetts Institute to control the training of the next generation of young mutants.