Fehn was born in Kongsberg, Buskerud. He received his architectural education shortly after World War II in Oslo, a crisis course that would later become an independent school under various names during the next decades, today known as the Oslo School of Architecture and Design. He quickly became the leading Norwegian architect of his generation.
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.
Schreiner House, Oslo, 1963