He was born in Trondhjem as a son of educator and major Anton Christian Føyn (1865–1940) and Olga Barth Nielsen (1870–1959).
He finished his secondary education at Trondhjem Cathedral School in 1918, and graduated from the Royal Frederick University with the cand.real.
He was a research assistant from 1923 to 1928 at the Royal Frederick University, and then under Max Hartmann at the Kaiser-Wilhelm-Institut für Biologie from 1929 to 1932.
The popular work Norges dyreliv ("Animal Life of Norway") came in four volumes between 1947 and 1950, edited by Føyn together with Johan Huus, Gudrun Ruud and Hagbart Røise.
[2] The staff Johannes Andenæs, Eiliv Skard, Johan Christian Schreiner, Harald Krabbe Schjelderup, Anatol Heintz, Odd Hassel, Ragnar Frisch, Carl Jacob Arnholm, Endre Berner and Bjørn Føyn were sent to Grini concentration camp.