The series does not tell Duncan's story in chronological order. Instead, the primary sequence of events is set in a present time that corresponds to the years during which the six seasons were filmed, i.e. 1992–1998, and extensive use of flashbacks is made to show Duncan's memories and their implications in present time.
It is established in the pilot episode "The Gathering", set in 1992, that Duncan is almost 400 years old and thus was born in 1592 and raised in Glenfinnan, Scotland. In the second episode, "Family Tree", Duncan's father and Chieftain of Clan MacLeod, Ian MacLeod, states that the newborn Duncan was a foundling given to his wife Mary to replace the stillborn son she had delivered, and raised to be his successor. The same episode also shows how Duncan is brought home fatally wounded in battle, dies in front of his father, and awakens with his wounds completely healed. Because his family believes his resurrection is a sign of witchcraft, he is cast out from his clan after being disowned by his adoptive father. The Watcher Chronicle of this episode states that it happens in 1622 during a dispute with the Clan Campbell. In "The Gathering", Duncan and Connor MacLeod explain how Connor, who had undergone a similar ordeal in 1536, found Duncan in 1625, told him about his immortality and became his mentor.
The series' various flashbacks show Duncan having many adventures across four centuries, including visiting Renaissance France and Italy ("The Hunters"), traveling to China ("The Road Not Taken"), witnessing the French Revolution, becoming a Sioux warrior ("The Gathering"), fighting in World War I ("For Tomorrow We Die") and II ("Mortal Sins"), and generally fighting for justice against evil. In 1815 at the Battle of Waterloo, he meets Immortal Darius who has rejected war, and follows his example by becoming a non-lethal participant in the later conflicts (for example, as a medic). The series especially focuses on adventures shared with friends Amanda and Hugh "Fitz" Fitzcairn.
During his life, Duncan meets and learns many things from many people that mold him to become a great Immortal. He meets many other Immortals: some become mentors, some become friends and some become enemies. Beginning as a young immortal, full of bravado (based on naivety) and also reckless and uneducated he gradually changes and matures as he travels the world until he becomes the wise, educated and compassionate character of present times. He is an expert in many subjects, fluent in many languages (Gaelic, English, Sioux, French, Italian, Russian, Spanish, Chinese, Japanese, German, Greek,and Arabic) and skilled in many forms of martial arts including Baguazhang, Xingyiquan, Wing Chun, karate, aikido, tae kwon do, judo, Brazilian jiu-jitsu, ninjitsu, Muay Thai, boxing, and Jeet Kune Do. He has had many occupations including soldier, bodyguard, newspaper editor, World War I ambulance driver, chauffeur, and World War II Resistance fighter. More recently he is an antiques dealer, a dojo owner and a part-time History teacher.
1992–1998 is a critical period for Duncan. Being the years during which the series was filmed, they are also the years in which the series' "present time" is set and thus depict many important events in his life. The pilot episode "The Gathering" shows Duncan returning to The Game after a period of semi-retirement with his girlfriend Tessa Noël and a meeting with clansman and mentor Connor MacLeod. Across the series, Duncan meets many old friends and enemies again, many old debts are repaid between them, and Duncan kills many Immortals. He loses people he cares about, including Tessa, the love of his life ("The Darkness"), Immortal friends Darius ("The Hunters") and Hugh Fitzcairn ("Star-Crossed"), and mortal friend Charlie DeSalvo ("Brothers in Arms"). He befriends Richie Ryan ("The Gathering"), mentors him after his First Death ("The Darkness") and teaches him the ways of the Immortals, then accidentally kills him in 1997 ("Archangel"). He meets Joe Dawson, who reveals to him that there is an organization known as The Watchers who secretly observes all Immortals ("The Watchers"). He also meets Methos, the legendary oldest Immortal ("Methos"), and eventually defeats the demon Ahriman ("Armageddon").
Because Highlander was a syndicated series including European producers, the first part of each season was filmed in Vancouver, British Columbia and the second part in Paris, France. This is why Duncan travels back-and-forth every six months.
In the feature film Highlander: Endgame (2000), it is revealed that Immortal Jacob Kell has spent the last four centuries killing all people close to Connor MacLeod because Connor had killed Kell's adoptive father. He also gathered a posse of Immortals who would overpower other Immortals, allowing Kell to then enter and take their heads. In the 1990s, Connor hid in a place called The Sanctuary, where Immortals were protected from The Game by the Watchers. Ten years after Rachel's death, Kell attacked the Sanctuary, and Connor was believed to be dead along with the other Immortals sheltered there. Duncan has visions of this evil act and investigates. He discovers Connor had been spared by Kell, so as to make his life even more of a misery. Kell wants to kill Duncan only to agonize Connor further, and given the number of Quickenings he has received, he would succeed in doing so. Therefore, Connor forces Duncan to kill him, to absorb his power, thereby enabling Duncan to defeat Kell. Duncan once again has to lose a friend but in the end he understands that Connor's death was necessary and it was the only possible way for him to kill Kell. Duncan buried Connor in the Scottish Highlands, in Glencoe, next to his first wife Heather's remains.
In the TV movie Highlander: The Source (2007), Duncan has married a mortal woman named Anna and has since separated from her. He is reunited with her at the Elder's monastery, and he joins in the quest to reach the Source after the death of Joe Dawson. As he grows close to the Source, he becomes mortal along with the other Immortals in the group (which includes Methos). Soon after making love to Anna, he overcomes the Guardian and enters the Source with her. She reveals to him that she is pregnant with their child.
Duncan is portrayed as being tall and brown-haired (usually long and/or in a ponytail). Like all Immortals, his aging process stopped at the moment of his First Death when he was 3 months away from his 30th birthday. Thus he's been looking like a man in his early thirties for four hundred years and will do so until, if ever, he is beheaded. His injuries heal very quickly and when he is killed by any means other than beheading, he revives some time later fully healthy again.
Duncan was raised to be the next chieftain of his clan and this fact influenced his personality. He has a strong moral conscience and a sense of nobility; he protects the weak and innocent and punishes the evildoers. He is known to be very loyal. He is referred to by more than one character as a "Boy Scout" and tends to see things in terms of black and white, being often criticized - sometimes in jest, sometimes not - for his rigid sense of honor and his determination to intervene in events that often place him in danger. He feels responsible for others and when he thinks people died because of him, he feels he has to avenge them. He mentors new Immortals even when he has doubts about them. Like all Immortals, he is unable to have children but he likes them and takes care of those he meets. He is very respected by both mortals and Immortals and considered by some to be one of the prime contenders for The Prize .
Duncan enjoys life. As Connor said, he has "all the fun and most of the good women". Although he has known many women, he shares his life with a few women he grows to love. Those who are mortal, he remains loyal to their memories and cherishes the moments he had with them. He is a perfectionist and keen to learn new things, be it reading, playing Shakespeare or fighting skills (see below). He is cunning and very difficult to defeat in combat because of his skill and because he has experienced most of everything life has to offer. He has a very good memory and can remember things that happened centuries ago.
As an Immortal, Duncan takes The Game with a grain of salt. He fulfills his duty and defends his head when he is challenged but he is not bloodthirsty and occasionally spares his opponents, especially female Immortals. Indeed, he does not hunt other Immortals for their heads, but he fights evil ones if they attack people he cares about. As he tells Alexei Voshin, "if you harm people I care about, I'll send you to hell the hard way". Unlike Connor, for example, he does not consider The Game his primary purpose in life and sometimes retires from it.
Duncan is not prejudiced against mortals (Indeed, despising mortals is an attribute of most evil Immortals). Duncan is generally very generous to mortals, and almost always tries to avoid hurting them. Being four hundred years old, he has seen thousands of them age and die and thus acutely, and sometimes bitterly knows how fragile they are. "I have mourned more friends than I can count. Death is part of my life." For this reason, he protects them, especially from evil Immortals and from the dangerous consequences The Game can have for them. He feels they have nothing to do with it and should not suffer from it. He doesn't usually tell them about his immortality except when he trusts them with his life. He never gets used to death, especially the death of his lovers. As he tells Tessa : "No matter how many years go by. Or how many times you say goodbye to those you care most about, when they leave, you..." [Tessa: "Die..."] "...yes. When they die, you're naked and alone." But it never prevents him from involving himself in life again.
After becoming a mentor to Richie, he matures into something of a father-figure, with a stronger, more confident outlook on life. After Richie's death, he becomes a darker, brooding character acutely aware of the fragility of life and his participation in The Game decreases.
Despite the fact that he was exiled from his own clan after his First Death in 1622, he remains proud of, and faithful to, his Scottish heritage. He usually introduces himself to other Immortals as "Duncan MacLeod of the Clan MacLeod" and fought for Scottish independence whenever he could, including the Battle of Culloden in 1746 ("Take Back the Night"), eventually trying to steal the Stone of Scone in 1950 ("The Stone of Scone"). In the book Scotland the Brave, despite its questionable canonicity, he remembers having helped to steal the Stone in a different version, closer to what really happened.
Executive Producer Bill Panzer thinks, "once you've hurt him, he doesn't forget. And Immortals have a long time to remember, and sooner or later you will cross his path."
Duncan considers himself "an old soldier" and fighting has been part of his life since the beginning, when he was raised to be a warrior. Being a perfectionist, he always seeks to improve himself and never misses an occasion to learn new skills. Aside from bare-knuckle brawling, he first learned swordsmanship in Scotland and the European continent. He then discovered martial arts during his travels in the lands of East Asia during the late 1700s. His sword is a katana, featuring a dragon's head, which was given by Samurai warrior Hideo Koto in 1778 during his travels to Japan. Duncan should have inherited the Claymore of his (adoptive) father, Chieftain of the Clan MacLeod, but because he was exiled the sword was instead kept within the clan. (In one episode he finds his father's claymore hanging as an antique in a Scottish tavern, and uses it to kill the immortal who killed his father some time after he was exiled). In fact, Duncan usually tries to avoid killing mortals even when he is forced to fight them, and goes out of his way to use non-lethal methods. Although MacLeod has fought in many armed conflicts and is skilled with firearms, he is reluctant to use them and rarely does so throughout the series.
Duncan is an excellent swordsman among Immortals. Although he is not one of the oldest Immortals in the Game, he has defeated numerous Immortals who are much older than himself, and many villainous Immortals, both male and female, with utterly fearsome reputations.
In the TV series continuity, Duncan is the favourite to win The Prize. In Highlander: Endgame it is established that Duncan has taken 176 heads. Here's the list of all of Duncan's known Quickenings:
1Gilliam beheads himself. Duncan's first Quickening.
²Duncan's first beheading.
³Beheaded by a propeller blade.
4Beheaded by a train.
5Beheaded by Watcher Jack Shapiro.
6Double Quickening with Methos who beheaded Silas at the same time.
Here's a list of all of Duncan's known Watchers:
Movies - Highlander: Endgame, Highlander: The Source
Episodes - "The Gathering", "Family Tree", "The Road Not Taken", "Innocent Man", "Free Fall", "Bad Day in Building A", "Mountain Men", "Deadly Medicine", "The Sea Witch", "Revenge is Sweet", "See No Evil", "Eyewitness", "Band of Brothers", "For Evil's Sake", "For Tomorrow We Die", "The Beast Below", "Saving Grace", "The Lady and the Tiger", "Eye of the Beholder", "Avenging Angel", "Nowhere to Run", "The Hunters", "The Watchers", "Studies in Light", "Turnabout", "The Darkness", "Eye For An Eye", "The Zone", "The Return of Amanda", "Revenge of the Sword", "Run For Your Life", "Epitaph for Tommy", "The Fighter", "Under Color of Authority", "Bless the Child", "Unholy Alliance", "Unholy Alliance Part Two", "The Vampire", "Warmonger", "Pharaoh's Daughter", "Legacy", "Prodigal Son", "Counterfeit", "Counterfeit Part Two", "The Samurai", "Line of Fire", "The Revolutionary", "The Cross of St. Antoine", "Rite of Passage", "Courage", "The Lamb", "Obsession", "Shadows", "Blackmail", "Vendetta", "They Also Serve", "Blind Faith", "Song of the Executioner", "Star-Crossed", "Methos", "Take Back the Night", "Testimony", "Mortal Sins", "Reasonable Doubt", "Finale", "Finale Part 2", "Homeland", "Brothers in Arms", "The Innocent", "Leader of the Pack", "Double Eagle", "Reunion", "The Colonel", "Reluctant Heroes", "The Wrath of Kali", "Chivalry", "Timeless", "The Blitz", "Something Wicked", "Deliverance", "Promises", "Methuselah's Gift", "The Immortal Cimoli", "Through A Glass, Darkly", "Double Jeopardy", "Till Death", "Judgement Day", "One Minute to Midnight", "Prophecy", "The End of Innocence", "Manhunt", "Glory Days", "Dramatic License", "Money No Object", "Haunted", "Little Tin God", "The Messenger", "The Valkyrie", "Comes a Horseman", "Revelation 6:8", "The Ransom of Richard Redstone", "Duende", "The Stone of Scone", "Forgive Us Our Trespasses", "The Modern Prometheus", "Archangel", "Avatar", "Armageddon", "Sins of the Father", "Diplomatic Immunity", "Patient Number 7", "Black Tower", "Unusual Suspects", "Justice", "Deadly Exposure", "To Be", "Not To Be"
Books - The Element of Fire, Scimitar, Scotland the Brave, Measure of a Man, The Path, Zealot, Shadow of Obsession, The Captive Soul, White Silence, An Evening at Joe's
TV Guide ranked Duncan MacLeod # 11 on its "25 Greatest Sci-Fi Legends" list.