He is known for his work in the Norwegian resistance movement, as editor of Verdens Gang and as a historical writer.
He was born in Solum as a son of merchant Nils Christian Christensen (1872–1957) and his wife Ellen Kristine Reiersen (1883–1953).
Christiansen participated for the Liberal Party in early negotiations with the occupying power, but as Germany seized tighter control in September 1940 Christiansen joined the Norwegian resistance,[4] and acted as a middle man between the Norwegian press and the Home Front.
A co-founder was Einar Gerhardsen,[1] and editors were Christensen, Olaf Solumsmoen and Olav Larssen.
[1] He was arrested by the Nazi authorities in late February 1941, for "spying", and sat for three months at Møllergata 19 and for five days in Grini concentration camp.
[5] Instead a new newspaper with roots in the Home Front was created, with Christensen as editor-in-chief: Verdens Gang.
In May 1945 he supported Paal Berg, head of Hjemmefrontens Ledelse in his endeavors to create a broad, non-partisan coalition.
Notable works included Fra verdenskrig til verdenskrig ('From World War to World War') and Okkupasjonsår og etterkrigstid ('Occupation Years and Post-war Era'), volumes eight and nine of the series Vårt folks historie ('Our People's History'), both issued in 1961.