Mark Thorn is the son of Richard Thorn from a previous marriage and cousin to Damien — with whom he gets along very well. The two boys enjoy the privileges of a prominent and wealthy family and attend Davidson Military Academy together. They both hold the rank of Cadet Staff Sergeant in the Bradley Platoon. Mark plays the bugle for the marching band.
A week after the events of The Omen we learn that Damien, now an orphan, has been sent to live with Robert Thorn's brother Richard in Chicago. Seven years later we see that he is very much a part of the family which consists of Richard, his second wife, Ann, Richard's aunt Marion, and Mark.
At school, While waiting to meet with their new platoon leader, Daniel Neff, bully 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 was devastated, all the more so because normally, in difficult times, Damien was the one he had always turned to. When Damien caught up with Mark in the woods outside the Thorn winter estate, Mark repeatedly refused Damien's offer to join him. Damien killed Mark by inducing an aneurysm in his brain, and act that he regretted deeply.
Richard was a caring and loving father to Mark, and Mark in turn thought highly of his father.
Mark and his stepmother Ann had a mother-son bond. Despite not being his biological mother, Ann played a strong parental role in Mark's life and approved of Mark's friendship with Damien.
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. Marion liked Mark nonetheless, and fought hard to have him and Damien separated, sensing something wrong with the latter.
Mark took an instant dislike to Teddy for his mockery of the Thorn family and Robert Thorn in particular. After Damien used his powers to subdue Teddy as he and Mark fought outside Sergeant Neff's office, Teddy ceased all attempts to antagonize Mark or Damien. The novel mentions he actually reversed himself, working to make amends and making a point of sitting with the Thorn cousins in class.