[1] Steenstrup worked for and later managed the insurance company Det norske gjensidige Skogbrandforsikringsselskab.
He was co-founder and manager of the insurance association Samtrygd Gjensidig Norsk Forsikringsforening.
He carried money from Government representative Anders Frihagen at the Norwegian legation in Stockholm to the underground movement in Norway.
Under the cover name "Cato" he coordinated Milorg's transport of refugees from Norway to Sweden, and was also involved in intelligence.
[4] In Sachsenhausen, he was selected as "postmaster", distributing post, a position which gave him a good overview of other prisoners.