Alnæs was born in Fredrikstad, as the son of headmaster  Johannes Jørgen Lauritz Alnæs (1835–1916) and Elise Martine Hansen (1851–1931).
Alnæs was born in Fredrikstad, the son of headmaster Johannes Jørgen Lauritz Alnæs, who lived from 1835 to 1916, and Elise Martine Hansen, who lived from 1851 to 1931.