He was born in Innvik as a son of vicar Mads Olsen Andenæs (1855–1942) and Signe Theoline Mydland (1883–1958).

He finished his secondary education at Stabekk in 1929, enrolled at the Royal Frederick University and graduated from there with the cand.jur.

He was hired as lecturer and research fellow at the University of Oslo in 1939, and took the dr.juris degree in 1943 with the thesis Straffbar unnlatelse.

[1] Andenæs applied to become docent in 1940 and professor in 1942, at the University of Oslo, but his candidacy was rejected both times due to the German occupation of Norway.

[3] The staff Carl Jacob Arnholm, Eiliv Skard, Johan Christian Schreiner, Harald Krabbe Schjelderup, Anatol Heintz, Odd Hassel, Ragnar Frisch, Bjørn Føyn, Endre Berner and Johannes Andenæs were sent to Grini concentration camp.

[4] In 1945 he was a consultant in the Office of the Director of Public Prosecutions, where the work with the legal purge in Norway after World War II had started.

He served as dean at the Faculty of Law from 1959 to 1960 and 1968 to 1969, prorector from 1960 to 1962 and rector from 1970 to 1972, and praeses of the Norwegian Academy of Science and Letters from 1977 to 1981.