Highlander: The Series was originally created to carry on directly from the movies with Paul playing the same character Christopher Lambert had played in the movie, but early in development, Paul requested that a new character be created to avoid direct comparisons with Lambert and to allow him to develop his own character. Consequently, Duncan is introduced in the pilot episode as Connor MacLeod's clansman and pupil, and the series focuses on his own life over four centuries.
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.