He finished his secondary education in 1933, took Oslo Commerce School in 1934 and took the cand.jur. degree in 1938. He worked as a junior solicitor in Mysen until 1940, then briefly as a police officer in Oslo before being deputy judge in Farsund from 1940 to 1941. In July 1941, one year after the German invasion and occupation of Norway, Aarestrup was arrested for an attempt to flee the country. He was first imprisoned in Kristiansand's Arkivet, and later sat in Møllergata 19 from 8 to 22 August and in Grini concentration camp until mid-October 1941.