Sean began his career as an agent of Interpol. Turning rogue in order to apprehend a serial killer named Arkady Rossovich, Sean was among the first to confront the mutant who would later become the savage Omega Red.
Later Banshee was forced into service for a malicious organization called Factor Three. Over time, Sean rid himself of their mind control with the help of the X-MEN. He then proceeded to aid Xavier's students in defeating Factor Three's leader, the Mutant Mind Master.
Banshee soon joined the ranks of the X-MEN, taking Professor Xavier's dream for his own.
In Moira MacTaggert, Sean found a sweet, caring woman who returned his love. Their relantionship flourished amidst their responsibilities to the Xavier Institute.
Banshee temporarily lost the use of sonic scream after pushing his powers to their limit while battling Moses Magnum. Feeling his contributions to the X-MEN limited by the absence of his mutant abilities, Sean left the mansion to work with Moira on Muir Island. Eventually, Moira and Sean separated to follow their own aspirations.
Banshee dicovered he had been missing out on a part of his life when he was finally united with his daughter. Unbeknownst to him, Theresa had been born years before, just prior to her mother's death in their native Ireland. Inheriting her father's X-Factor, a sonic scream, Siryn later became leader of the mutant team X-FORCE.
After witnessing their apparent deaths at the hands of the Adversary, Banshee refused to believe that the X-MEN had perished. Once again donning an X-uniform, he was successful in finding and regrouping his former teammates.
Finally ready to pass on his experience and wisdom, Banshee became headmaster at The Xavier Scholl for Gifted Youngsters. Here he continued to train and instruct Generation X, the newest chapter in the Xavier legacy.