He was born in Selje as a son of district physician Johan Nicolay Cappelen (1818–1890) and Elisabeth Brunchorst Ravn (1825–1906). His brother Axel Hermansen Cappelen was also a physician. In 1881, he married Katharina Margrethe Steen (1859—1915). They had one daughter, Astrid, and two sons, Johan and Christian,Norske slægter (1912), p. 81 as well as several children who did not survive infancy.
