James Horton was a Watcher and the leader of the Hunters, a faction of renegade Watchers that sought to wipe out all Immortals, whom they considered abominations that had to be destroyed. From 1981 to 1985 he had been the Watcher of The Kurgan, one of the most brutal Immortals ever recorded, which may have influenced Horton's belief that Immortals had to be eliminated. Additionally, his assignment following the Kurgan was Blake Wilmington, who according to Watcher records, perpetrated a brutal massacre at an amusement park in 1988. Horton was evaluated by the Watchers Psychology and Counseling department, but was at the time judged to have no lingering trauma over witnessing such events.
James Horton was first introduced in the first season finale episode, "The Hunters" as the leader of the men who enter Darius's chapel and beheaded him. Those men, called the Hunters, then kidnapped MacLeod's friend, fellow Immortal Hugh Fitzcairn. Horton later set up a plan to meet MacLeod's lover Tessa Noël and check a wound she had on her hand to see if she was immortal. Tessa was frightened and bewildered by this encounter. In the meantime, Fitzcairn had been brought to the Hunters' headquarters. Horton and his accomplices "study Fitzcairn as one would a large, interesting insect," then shot him with a crossbow to see if he will revive, proving that he was an Immortal. When Fitzcairn awoke, Horton said, "I've often wondered how much pain an Immortal could bear before he went mad."
MacLeod searched for the Hunters' hiding place while Horton addressed his men: "We have won a great victory. We have destroyed a malignant evil that has walked this earth in the form of a man for the last two thousand years. People do not cheer us, people don't even know we exist, but we know the battle we fight. We know the evil we must destroy. I would like to send you back to your homes... to your lives... but our work here is not finished." Horton tells Fitzcairn, "You are an abomination before nature and in the eyes of man. There is no glory but ours. No destiny that is not of our making." Horton was about to guillotine Fitzcairn when MacLeod entered. In the ensuing fight, Horton told MacLeod, "When you kill me, others will come. To continue what we have begun. We've always understood. We will never be dominated. We know about the Gathering. It's about power. There is nothing greater that the power of man. Nothing. You must die. All of you."
In "The Watchers" after MacLeod met his Watcher, Joe Dawson, Horton remarked to Dawson, "You spoke with one of them. That's never happened before." When Robert Tucker, one of his Hunters who was also the fiancé of his daughter, Lynn Horton, expressed doubts about the Hunters, Horton answered, "It doesn't matter if they're good or evil, Robert. They're here to dominate us. They fight for their right to rule us. And one day, that is what they will do." When Robert said he wanted to leave the Hunters because MacLeod spared his life, Horton killed him. Horton then mounted a trap to behead MacLeod, but Dawson confronted him, saying, "What you are doing is totally wrong. It betrays everything we stand for. They aren't all evil." Horton answered, "I felt like a doctor watching a cancer grow."
MacLeod arrived and Horton admitted to having killed Robert, saying, "Sometimes in war innocents get harmed. I believe that the generals call it collateral damage." MacLeod was about to kill Horton but stopped because Lynn begged him not to. Horton was infuriated and shot MacLeod in the back. MacLeod was able to stab him with his sword before collapsing dead. When he revived, the others had disappeared.In "Unholy Alliance: Part One" Horton concluded an agreement with Immortal Xavier St. Cloud, in which Horton provided mercenaries who shot Immortals dead so that Xavier could safely behead them. Horton also used Xavier's Watcher, Barton, to manipulate Dawson so that Dawson told MacLeod where Xavier was, who was then waiting for him with his mercenaries. MacLeod came as planned, but his friend Charlie DeSalvo had followed him and witnessed the fight between MacLeod and Xavier, and MacLeod being mortally shot by Horton, before being shot himself.
MacLeod revived and broke open Horton's grave in the Dawson family crypt but it was empty, and MacLeod was then approached by Horton. MacLeod moved to attack him, but Horton warns him, "Holy Ground, MacLeod! Shame on you... You're forgetting the rules. I tried to get Xavier to come but even he wouldn't kill here." When MacLeod asked if Horton does this to feel powerful, Horton replied, "This is not about me, MacLeod. This is about you, you and your kind. You're an abomination." Then Horton boasted, "I'm the man you can't kill. Frustrated? I know I would be in your position. How would you like to kill me? ...with your sword? No, I think you'd prefer to do it with your bare hands, feel the life drain away from me." Eventually Horton hinted that Dawson knew everything and escaped. Later, MacLeod watched Dawson join Horton on a boat, and then cornered Dawson to hear the truth from him. Dawson explained, "After you fought with Horton, I had to take him to the hospital. He was dying. I made sure he was through with the Watchers, MacLeod. No one was to contact him, and not even his own people. He was an outcast. I've known the man for twenty-five years, he's my sister's husband. I couldn't just let him die. He said he just wanted to leave the country, live quiet." Dawson said he did not tell this to MacLeod "because I knew you'd kill him." MacLeod was hurt that Dawson lied to him and walked away.
In "Unholy Alliance: Part Two" Horton and Xavier were still plotting MacLeod's death together. During their conversation Xavier put a hand on Horton's shoulder and Horton shows disgust at this physical contact with an Immortal. He remarks, "It would give you great pleasure to kill Duncan MacLeod, would it not ?" When MacLeod enters, Horton got a helicopter to allow Xavier to flee the ensuing fight. Later in Paris, when MacLeod found their hiding place, Horton tried to flee again on a boat, but Dawson stopped him. Despite Horton's claim that "we're family," Dawson shot him and Horton fell in the river. Horton was next seen in the episode tag, watching MacLeod from a distance and apparently unharmed.
In "Counterfeit: Part One" Horton set up an elaborated trap to kill MacLeod. He uses Pete Wilder, to befriend Richie and make MacLeod suspicious, which spread discord between the two. At the same time, he has murderer, Lisa Halle kidnapped and her appearance altered by plastic surgery so that she looked like Tessa. While she recoverd from the operation, he mused, "True greatness is completing a task that others have found impossible. I am going to do something that many have tried and that none have ever achieved and that only a handful have lived to know about. I'm going to destroy Duncan MacLeod. And, every day, for the rest of my life, I'm going to remember and relish it. In a way, it's sad, because whatever else I do, I shall never equal this."
In "Counterfeit: Part Two" Dawson mentioned that he and Horton designed the Watcher's computer security system together "to be bullet-proof" and that it used thumb print to get in. MacLeod rationalized that Horton might have survived Dawson's shot by wearing a ballistic vest. Horton tried to kill Dawson but Richie took the bullet instead. Horton waited for Lisa to kill MacLeod on Tessa's grave. Horton entered after hearing a gunshot, and tried to behead MacLeod but MacLeod stopped him. Horton killeds Lisa and ran for his life, but MacLeod caught up with him and killed him with his own knife.
Horton was possessed of fanatical conviction, and was gifted with a keen intelligence and perception. He was methodical and patient.