Career
He was elected to the Norwegian Parliament in 1848, 1851 and 1854, representing the constituency of Stavanger. He worked as a merchant in that city. Johnsen was also a writer
He founded the magazine Almuevennen in 1848, and wrote enlightening books such as Frihandelen (1860) and Reise i Universet (1885).
He published an encyclopedia in three volumes, Norsk Haandlexikon for almennyttige Kundskaber, and a short version Haandbog til Brug for alle: kortfattet Konversationslexikon.