He was born in London as the son of military officer Isak Tobias Isaachsen and Fredrikke Andrea Rude. He was a great-grandson of Isaach Isaachsen, brother of scientist Haakon Isaachsen and scientist Isak Isaachsen, nephew of painter Olaf Isaachsen and cousin of painter Herman Willoch and naval officer Odd Isaachsen Willoch. In 1861 he married Asta Lie, a daughter of Jonas Lie.