He was born in Trondheim, the son of director Christian Fredy Michael Jahn (1837–1914) and Elisabeth Wilhelmine Wexelsen (1853–1930).

He finished his secondary education at Trondheim Cathedral School in 1902 graduated from the Royal Frederick University with the cand.jur.

[3] In 1940 he was a member of the Administrative Council, which tried to maintain Norwegian political governance despite the German invasion and occupation of Norway.

[1] He instead became Minister of Finance and Customs of the Norwegian interim government, Gerhardsen's First Cabinet, on 25 June.

[1] In 1965, UN Secretary General U Thant was informed by the Norwegian Permanent Representative to the UN that he would be awarded that year's Nobel Peace Prize and asked whether or not he would accept.

The disagreement lasted three years and in 1966 and 1967 no prize was given, with Jahn effectively vetoing an award to U Thant.