He was born in
Kristiania as the son of banker Per Arntzen (1864–1924) and his wife Aasta Høst (1869–1945), He was a distant descendant of
Andreas Arntzen. He took the
cand.jur. degree with excellent grades in 1920, and then worked as a deputy judge. From 1922 to 1924 he studied in
France. He then settled in Kristiania/Oslo as a lawyer, and gained access to
Supreme Court cases in 1927. He also worked with law candidates at the
University of Oslo, and was a prominent member of the
Norwegian Bar Association. ==World War II==