Hans Johansen was born in Riga, Governorate of Livonia, Russian Empire (Latvia nowadays) to Danish parents.
Hans Johansen moved to Kamchatka in 1928, and shortly thereafter settled on the Commander Islands as a scientific leader of fur animal exploitation there.
He succeeded in building a small group of ornithology students, who were sent off to investigate the bird fauna of hitherto unexplored areas of Western Siberia.
During his years as a professor in Tomsk, Hans Johansen made frequent study visits to the bird collections of the Russian Academy of Sciences in St Petersburg.
Here he was employed at the Zoological Museum of the University of Copenhagen and headed its bird ringing centre 1943–1960.
He repeatedly visited the USSR, where he participated in conferences and provided communication for Russian and European ornithologists.
At the end of his life he bought a cottage on the isle of Læsø, the barren landscape of which reminded him of Siberia.