In the book, Miller is a member of the Ulster Liberation Army. He is attacking the Prince of Wales car when he is stopped and injured by Jack Ryan and is then arrested. However he breaks out of Jail and travels to America, where he attempts to kill Cathy and Sally Ryan, but fails. During the attempted murder Miller does kill a highway patrolman. Then, he and other members of the ULA break into Ryan's house when he is hosting the Prince and Princess of Wales along with his friends the Jacksons. Ryan and his friends escape killing some of the terrorists. As Miller and the rest of the ULA try to escape on a boat, they are arrested by Ryan and a group of Marines. Ryan wants to kill Miller but does not, deciding it's not worth it. Miller is handed over to the authorities and, along with the rest of the ULA, is executed for his crimes.
In the movie Patriot Games, Sean Miller is played by Sean Bean. He and his brother Paddy Miller attack Lord Holmes motorcade, and Ryan kills Paddy stopping them. This is the origin of Sean's obsession with Ryan in the movie.
In the movie as in the book, Sean does not talk to Ryan, but in the movie, Sean does yell at Ryan during his trial, instead of quietly staring. He is rescued by Kevin O'Donnell and the others and kills the Irish officer investigating the assassination.
In the movie, O'Donnell says he gave Miller the trip to America "for Paddy," but has to almost beat sense into Miller when it is revealed that the Ryans are alive. Miller contents himself with making threatening phone calls to the Ryan's home--something he never does in the book.
When Dennis Cooley comes to the ULA camp and demands to be brought on the next mission, Miller shoots a gun at his feet to test him. When he fails this, Miller kills Cooley, with O'Donnell's tacit approval.
Miller is part of the attack on the Ryan's home in the movie, and chases after Ryan in the boat with O'Donnell. When it is revealed that Ryan is alone in the boat, O'Donnell insists that they turn around to capture Lord Holmes, but Miller kills him in order to keep after Ryan. He jumps on the boat and uses an anchor to attack Ryan, but later impales himself on this same anchor.
- Sean Miller only appears in Patriot Games starting English actor Sean Bean as Miller.