He was born in Kristiania as a son of Jakob Peter Thommessen and Maja Johannessen.
[1] He took his examen artium at Aars og Voss skole in 1908, and then graduated from the Royal Frederick University with the cand.jur.
He was a deputy judge in Aker from 1913 to 1916 and a junior solicitor from 1916, until taking the lawyer's credentials in 1919.
In cooperation with Tor Skjønsberg he had regular contact with the Norwegian legation in Stockholm, securing several monetary loans through contacts with the government-in-exile representatives stationed in Stockholm.
[1] He was chairman of the Norwegian Museum of Cultural History from 1948 to 1966, and a board member of Gyldendal Norsk Forlag from 1941 to 1965, Elektrokemisk from 1952 to 1965 and Orkla Grube-Aktiebolag from 1956 to 1957.