[1][2] Johnsen became a student a 1896; he graduated with a master's degree in 1898, and in 1906 he received a PhD with his dissertation De norske stænder (The Norwegian Estates).

[1][2] Johnsen and Halvdan Koht influenced Norwegian historiography in the 1900s through their views that there was no constitutional equality between Denmark and Norway after the introduction of the absolute monarchy in AD 1660.

[3] Together with the teachers Lorens Berg from Andebu and Jacob Aaland from Nordfjord, as well as Edvard Bull Sr., Johnsen was central to the new local historical movement that arose after the dissolution of the union.

This led to the establishment of the National Society for Rural and Urban History (Landslaget for bygde- og byhistorie) in 1920, where Johnsen served as chairman from its beginning until 1945.

[4] During the Second World War, the nationally conservative Johnsen was head of the newly created Institute for Medieval Texts (Institutt for middelaldertekster).