Mark was the son of Richard Thorn and his first wife, and therefore the stepson of Richard's second wife, Ann. He was also the cousin of Damien, whom he got along very well with and was enrolled in Davidson Military Academy with.
The day after Aunt Marion dies of a heart attack, Sergeant Daniel Neff is appointed the new leader of Mark and Damien's platoon at the academy. A day later, after Neff visits Mark in the former's office, cadet Teddy mocks the Thorn family in front of Mark. Mark punches Teddy in retaliation, and Teddy then starts beating Mark up until Damien intervenes. Damien causes Teddy to go into a fit of terror, to Mark's awe, until Neff breaks Damien's focus.
To take their minds off of what Damien just did, Mark and Damien decide to play rugby in the academy field. Days later, on Mark's birthday, the Thorns have a large birthday party for Mark. The day after, Mark, Richard and Damien are playing ice hockey on the frozen-over lake, when Richard's old friend Bill Atherton falls through the ice and is killed.Some days later, Damien takes Mark and several other academy cadets on a field trip to a Thorn Industries chemical plant. There, when the machinery breaks down, releasing toxic fumes, Mark, Damien and the other cadets are taken to hospital; but are allowed to leave and return home when they do not suffer any severe effects from the chemicals.
Later one night, while the Thorns are watching a movie at home, Dr. Charles Warren visits to speak to Richard in private. Mark overhears Charles' conversation with Richard, in which Charles theorizes to a denying Richard that Damien is the Antichrist. Mark discovers from a Bible and what he previously saw Damien do to Teddy that it is true.The next morning, Damien follows Mark into the woods near the Thorn residence, and Mark tries to avoid him. When an afraid and upset Mark confronts Damien about his unholy origins and what Charles said, Damien admits first reluctantly and then proudly to being the Son of Satan. He also admits that Mark is like a brother to him, and asks him to join him. When Mark refuses, Damien introduces an aneurysm into Mark's brain, painfully killing him.
Mark and Damien got along well and had an almost brotherly bond. When Mark discovered that Damien was the Antichrist, he came to fear Damien and refused his offer to join him. Mark also appeared to be deeply upset when he discovered that Damien was the Son of Satan.
Richard was a caring and loving father to Mark.
Mark and his stepmother Ann had a mother-son bond.
Mark was less fond of Sergeant Neff than Damien; regarding Neff as being no less strict than the platoon's previous commanders.
Mark regarded Marion as being very odd.
Mark had a strong hatred for Teddy for the latter's mocking the Thorns.