He is best known for his role as Aragorn in Peter Jackson's The Lord of the Rings film trilogy. He was a last-minute replacement for the role originally meant for Stuart Townsend, and wouldn't have taken the part if it hadn't been for his son's enthusiasm for J. R. R. Tolkien's trilogy. With part of the money he received for the film he founded the Perceval Press publishing house - named for the knight from the legend of King Arthur - to help other artists by publishing avant-garde works that might not find a home in more traditional publishing venues. He has spoken out against militarism and U.S. foreign policy.

After several years of experience in live theater, he made his first movie appearance playing an Amish farmer in Peter Weir's Witness. (Mortensen had actually been cast in two prior films - Swing Shift and The Purple Rose of Cairo - but his scenes in both of these films were deleted from the final cuts.) He has also appeared in Jane Campion's The Portrait of a Lady, Sean Penn's Indian Runner, Brian DePalma's Carlito's Way, Tony Scott's Crimson Tide, Ridley Scott's G.I. Jane, Tony Goldwyn's A Walk on the Moon, Philip Ridley's Reflecting Skin, Andrew Davis's A Perfect Murder, and David Cronenberg's A History of Violence.

Mortensen's performance in Bent at the Coast Playhouse, Los Angeles, won him a Dramalogue Critics' Award. Coincidentally, the play, about homosexual concentration camp prisoners, was originally brought to prominence by Sir Ian McKellen, with whom Mortensen co-starred in The Lord of the Rings. In 2004, he starred as Frank Hopkins in Hidalgo, the story of a Pony Express courier who travels to Arabia to compete with his horse, Hidalgo, in a dangerous race for a massive contest prize. Mortensen himself is very fond of horses, and shows such in his book The Horse Is Good. In fact, Mortensen bought the horse which played Brego in The Lord of the Rings movies (Roheryn in the books), which is Aragorn's steed; as well as Oscar, one of the horses who played Hidalgo. He also purchased the stallion that played Arwen's horse, and gave it to the stunt woman who rode the horse in place of Liv Tyler.

Mortensen is also an ice hockey fan, particularly of the Montreal Canadiens. He also likes soccer and is a big fan of the Danish national team and the Argentinian team San Lorenzo de Almagro. In 1992 Viggo went to Ireland during a break in shooting, without the consent of the production company, to watch Denmark play the semi-finals. He is also a fan of the New York Mets

In the Two Towers DVD extras, the film's swordmaster Bob Anderson described Mortensen as "the best swordsman I've ever trained". In the DVD extras for A History of Violence, David Cronenberg relates that Mortensen is the only actor he'd come across who would come back from weekends with his family having bought items to use as props on the set.

Mortensen's bibliography includes

Ten Last Night (1993), his first collection of poetry; Recent Forgeries (1998), ISBN 1-889195-32-4 5th Edition, documents Viggo's first solo exhibition and includes a CD with music and spoken-word poetry. Introduction by Dennis Hopper; Errant Vine (2000), limited edition booklet of an exhibit at the Robert Mann Gallery. Only about 300 were published at the time of the exhibition so it is a very rare book; Hole in the Sun (2002, ISBN 0972143610), color and black and white photographs of a back yard swimming pool; SignLanguage (2002 ISBN 1889195499), a catalog from an exhibition of his works, combining photographs, paintings, and poetry into a multimedia diary of his time in New Zealand while filming The Lord of the Rings: The Fellowship of the Ring; Coincidence of Memory (2002, ISBN 0-9721436-0-2 Third Edition, in this book, the artist combines photographs, paintings, and poems that cover his artistic output from 1978 to 2002; Mo Te Upoko-o-te-ika/For Wellington (2003), ISBN 0-9721436-8-8, a book to accompany the joint exhibitions at Massey University and the Wellington City Gallery during the premiere of The Lord of the Rings: The Return of the King; 45301 (2003), ISBN 0-9721436-3-7, Abstract images, fragments and phrases from poems create this photography book. Many of the photographs were shot during travels to Morocco, Cuba, and the northern plains of the United States; Un hueco en el sol (2003), a small booklet was published to accompany the exhibition "Un hueco en el sol" at the Fototeca de Cuba in Havana. In Spanish; Miyelo (2003), ISBN 097214367X), a series of panoramic photographs of a Lakota Ghost Dance; The Horse is Good (2004), ISBN 0-9747078-1-3, a photography book partly shot during his work on the film Hidalgo about horses as partners, teachers, and fellow travelers. Images from Morocco, South Dakota, Montana, California, Iceland, New Zealand, Denmark, Brazil, Argentina. Mortensen spent some childhood years in Venezuela, Argentina, and his father's native Denmark, before returning to high school in Watertown, New York (where he was captain of the swim team). He graduated from St. Lawrence University in 1980. He is fluent in English, Spanish and Danish. He also speaks French, Norwegian and Swedish reasonably well.