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, and wouldn't have taken the part if it hadn't been for his son's enthusiasm for J. R. R. Tolkien's trilogy.
In the Two Towers DVD extras, the film's swordmaster Bob Anderson described Mortensen as "the best swordsman I've ever trained".
His voice is featured on The Lord of the Rings: The Return of the King soundtrack - he sings 'Aragorn's Coronation', the words by Tolkien but the music composed by Viggo himself. His poems are written in English, Danish, and Spanish.
During the scene in The Two Towers in which Aragorn, Legolas and Gimli find the band of Uruk-hai that had captured Merry and Pippin in Amon Hen, Aragorn, in his sorrowful anger, kicks an Uruk helmet. In the filming of this, Viggo broke three of his toes kicking the helmet. They used the scream of pain he uttered in the final film, as they thought it added an effect to Aragorn's sorrow.