After the German occupation of Norway, Bryhn served as assistant of Chief of Police of Oslo Kristian Welhaven.

When Welhaven was removed from his position, Bryhn and others started undercover intelligence work within the police corps.

He fled to Sweden, where he built up surveillance and security service at Norwegian legation in Stockholm.

His main task was to control refugees and couriers from Norway, and he eventually became a controversial person at the legation.

From 1957 to 1966 he was head of the Police Surveillance Agency (Norwegian: Politiets Overvåkningstjeneste, POT).