Viggo's father was Danish, his mother was American, and his maternal grandfather was Canadian.
Mortensen was born on October 20, 1958, in New York City, to Grace Gamble (Atkinson) and Viggo Peter Mortensen, Sr. His parents met in Norway. They wed and moved to New York, where Viggo, Jr. was born, before moving to South America, where Viggo, Sr. managed chicken farms and ranches in Venezuela and Argentina. Two more sons were born, Charles and Walter, before the marriage grew increasingly unhappy. When Viggo was seven, his parents sent him to a strict boarding school, isolated in the foothills of the mountains of Argentina. Then, at age eleven, his parents divorced. His mother moved herself and the children back to her home state of New York.