Sean Cassidy was born and raised in Ireland as the heir to Cassidy Keep, his family's ancestral castle. Like many, his mutant powers manifested in his adolescence, but Sean hid them from everyone. He gained a Science degree from Trinity College, Dublin and became, first a detective, then an inspector for Interpol, an international police force.

During this period of his life he married Maeve O'Rourke, over whom he and his cousin, Tom (the only person who knew that Sean was a mutant), had been rivals. While Sean was away on a secret long-term mission, Maeve discovered that she was pregnant. She had her child, a daughter named Theresa, before her husband returned home. After the birth of Theresa, Maeve decided to visit some relations in Northern Ireland and was killed by a terrorist bomb. Theresa's body was not found and it was assumed that she was also dead. However, Tom Cassidy had rescued the child and planned to raise her himself in the hope that she would develop mutant powers that he could use to his own benefit.

At the end of his mission, Sean discovered that his wife was dead, but was not told that he had also had a daughter who had been (supposedly) killed along with her mother. Overcome by grief, he left Interpol and worked for the NYPD in America for awhile. However, his grief continued to affect him and he left the police force, becoming a freelance operative. He gradually became involved in crime (although he was never arrested) leading to his association with Factor Three.

Sean Cassidy The Changeling, a mutant belonging to Factor Three, invited Sean to join the group. When Sean refused, Factor Three kidnapped and enslaved him. They gave him a new code name, Banshee, and forced him to participate in their criminal activities. This led to a clash with the X-Men in New York. The X-Men captured Sean and Professor Xavier was able to disarm the headband Factor Three had used to enslave him. In return, Sean told the X-Men everything he knew about Factor Three. However, Sean was captured by Factor Three again, and set free again. He helped the X-Men to destroy the groups' plans for world domination. He also helped the X-Men to defeat the Mutant Master, who was exposed as an alien.

Leaving his temporary allies, Sean wandered for a few years, having run-ins with the second Secret Empire and the Sentinels. When the original X-team disappeared, Professor X invited Sean to join a new generation of X-Men to search for the first. Sean accepted and stayed with the X-Men for quite some time. Through the team he met Dr Moira MacTaggert, who he fell in love with. The pair were involved in a relationship until her death.

After a battle with Moses Magnum in Japan, Sean damaged his vocal chords and seemingly lost the use of his sonic scream. Returning to Muir Island, he and the X-Men fought Proteus, Moira's son. This battle made Sean realise that he wanted to stay with Moira permanently instead of visiting her occasionally as he had been doing. He left the X-Men, although he continued to aid them when necessary, and stayed on Muir Island. During his stay on the island, the X-Men fought Black Tom Cassidy, and his accomplice, Theresa Rourke. After their defeat at the hands of the X-Men, Tom told Theresa that Sean Cassidy was really her father and she left Tom to meet him.

Theresa, codenamed Siryn, had developed powers similar to those of her father. After their first meeting, Sean gained custody of her, but their relationship was quite distant. Theresa joined, and eventually became the leader of, X-Force and continued to see Sean intermittently. It is only recently that the pair have developed a deeper relationship.

Sean left Muir Island to become the co-head of Xavier's next generation of mutants - Generation X. He, along with Emma "White Queen" Frost, was responsible for teaching the teenage group how to deal with their powers. While with the team, Sean battled various enemies and developed something of a relationship with Emma. Their burgeoning romance vanished, however, when the White Queen agreed to trade one of the students (Penance) to Emplate in exchange for the safe returns of the other students. This amazed and appalled Sean, causing him to return to something of his old mistrust for the former leader of the Hellions.

After the death of Moira and the disbanding of Generation X, Sean founded X-Corps, a militant group dedicated to the protection of mutants, especially those in Europe. The group captured Sean Cassidy mutants on the wrong side of the law and used mind-control to make them work for X-Corps. It was involved in many demonstrations and riots where it often used force, although never fatally. Sean's methods and ideas became more and more extreme, causing concern from both the X-Men and his former students, several of whom worked with him. In an attack planned to liberate one of his prisoners, Sean's throat was slit by Mystique, who left him to bleed to death. He did not die, however, and was rescued by the X-Men. He has since recovered the use of his powers.