Uther Pendragon (played by Anthony Head) was the King of Camelot; later dies in series four. He despises the use of magic and orders the execution of anyone caught using sorcery, thus forcing Merlin to hide the use of his powers. Uther has a son named Arthur and a legal ward named Morgana. He has a very tyrannical attitude as king and his son is just like him in many ways. He was once a knight but became King many years ago. In "The Dragon's Call", he was threatened by Mary Collins after he executed her son. At the end of the episode, she nearly kills Arthur, but Merlin saves him. King Uther then makes Merlin Arthur's manservant. Uther has a deep fear and hatred of magic, and sometimes mentions an event that caused his fears. This hatred is so strong that he has condemned at least three people to death (Mary Collins's son, a Druid, and the Druid's child apprentice Mordred) solely for the crime of knowing how to use magic, not because they ever used it to cause harm.

He is particularly afraid of Nimueh and it is hinted that he has a personal connection to her and that she caused his hatred of magic. In "Excalibur" it is revealed that his wife Igraine, whom he loved, was barren and so he begged Nimueh (who was his friend and the official court sorceress) to use magic to give him a son. His wife died during Arthur's birth- as a consequence of magic, a life could not be given without another life being taken-, and afterwards he blamed magic for what had happened, banishing Nimueh from the court and killing "her kind." His wife's brother, Tristan, blamed him for her death, and was killed in single combat with Uther. He is proud of his son but worries for Arthur's future as king, and hides this behind overly controlling and critical behaviour towards Arthur, leading Arthur to believe he is a "big disappointment."

He thinks Merlin is an idiot, but is grateful for his loyalty towards his son, telling Merlin to "look after him" when Uther was expecting to be killed by the Black Knight. Though he swore to protect Morgana and seems quite fond of her, he claims he will break his promise to protect her without a second thought if she betrays him. Although Uther is aware of Gaius's past use of magic, he seems to consider it irrelevant in his relationship with Gaius in the present, regularly confiding in Gaius about various issues even if his prejudice against magic commonly blinds him to the wisdom of Gaius's advice.

According to Anthony Head, Uther was to become "darker and badder" in season 2, although on many occasions he demonstrated a greater humanity than he had previously. Although he still ordered the execution of any magical practitioners he discovered in Camelot, as well as ordering an excessively high tax while under the influence of 'Lady Catrina'- really a troll posing as a human- that Arthur protested the people could not afford to pay, he later demonstrated his essential humanity when Merlin and Gaius prepared a potion that simulated the appearance of death to make Uther think that Arthur was dead; tears of true remorse proved sufficient to end Catrina's spell and make Uther see her for what she really was. He also appeared genuinely regretful when he learned that the witchfinder's accusations that Gaius was still practising magic were false, actually going down to Gaius's chambers so that he could apologise for Gaius's treatment at the Witchfinder's hands, and simply listening to Gaius's statement that his vendetta against magic would lead to trouble in future where he would have reacted violently to any other man for speaking to him in such a manner. His essentially good- if primarily misguided- nature was also shown on other dealings with Gaius, such as when he learned that Gaius had smuggled Morgause, Morgana's half-sister, out of Camelot after her birth despite the implied order to kill her, further demonstrating his friendship with Gaius as he simply listens to his friend rather than becoming angry at the defiance.

Following Morgana's return to Camelot, Uther is briefly seemingly stricken with madness when he begins to have hallucinations of the women and children whose deaths he had caused during the purges- including a vision of his own wife-, but Merlin and Gaius were able to break the enchantment. Although Merlin is aware of Morgana's treachery, he is forced to remain silent because Uther's devotion to Morgana meas that he would never believe Merlin's claims.

Although he tries his best to keep tight control over his Kingdom and those around him, he remains completely oblivious to the fact that Morgana possesses the magic he so despises, that his son Arthur has fallen in love with a servant (Gwen) and is becoming close friends with a warlock (Merlin) and that his trusted friend Gaius is protecting both Merlin and Morgana's magical secrets.