He was born in
Oslo, Norway. He was the son of Eivind Isaachsen Willoch (1866–1926) and Marie Wenneberg (1873–1951). He was the grandson of painter
Olaf Isaachsen. He was a
second cousin of both national aviation director
Erik Isaachsen Willoch and Norwegian Prime Minister 1981–1986
Kåre Isaachsen Willoch. Sigurd Willoch took the
mag.art. degree in 1925 from the University of Kristiania. He worked as a
research fellow from 1928 to 1933, taking the
doctorate in 1932. He was the art critic for the newspapers
Aftenposten from 1935 to 1942 and
Morgenbladet 1945–46. Willoch wrote about 19th century Norwegian art and artists, including works on Olaf Isaachsen,
Edvard Munch, Hugo Lous Mohr (1889–1970), August Cappelen (1827–1852) and Thomas Fearnley (1802–1842). He also wrote about the Art Society in Oslo and the National Gallery of Norway. He was employed as a curator at
Akershus Castle in 1937 and held this position until he took over as director of the
National Gallery of Norway in 1946. He was the director of the National Gallery from 1946 to 1973 and on its board from 1952–1976. He represented Norway in the Scandinavian Museum Association, Comité International d'Histoire de l'Art (CIHA), and
International Council of Museums. ==Honors==