Fehn was born at Kongsberg in Buskerud, Norway. He was the son of John Tryggve Fehn (1894?1981) and Sigrid Johnsen. He received his architectural education at the Oslo School of Architecture and Design in Oslo. He entered his course of study in 1946 and graduated during 1949. Among other instructors, he studied under Arne Korsm. In 1949, Fehn and architect Geir Grung won the competition for the Museum Building for the Sandvig Collections at Maihaugen in Lillehammer. In 1950, Fehn joined PAGON (Progressive Architects Group Oslo, Norway). The group, which was led by Arne Korsmo, had the goal of implementing and promoting modern architecture. In 1952-1953, during travels in Morocco, he discovered vernacular architecture, which was to deeply influence his future work. Later he moved to Paris, where he worked for two years in the studio of Jean Prouvé, and where he knew Le Corbusier. On his return to Norway in 1954, he opened a studio of his own in Oslo. At the age of 34, Fehn gained international recognition for his design of the Norwegian Pavilion at the 1958 Brussels World Exhibition. In the 1960s he produced two works that have remained highlights in his career: the Nordic Pavilion at the Venice Biennale (1962) and the Hedmark Museum in Hamar. Other notable works include the Norwegian Glacier Museum at Fjærland and the National Museum of Art, Architecture and Design in Oslo. He was a professor at Oslo's School of Architecture from 1971 to 1995 and principal from 1986-1989. He additionally lectured throughout Europe including at Paris, Stuttgart and Barcelona. He also lectured in the United States at the Cranbrook Academy of Art in Bloomfield Hills, Michigan, Cooper Union in New York City and Massachusetts Institute of Technology in Boston.