He was born in Skien as a son of customs surveyor and politician Johan Christian Tandberg Castberg (1827–1899) and his wife Hanna Magdalene Frisak Ebbesen (1839–1881).
[1] He finished his secondary education in 1894 and graduated from the Royal Frederick University with the cand.jur.
He chaired the regional branch of the Norwegian Bar Association from 1926 to 1936.
He was a member of Gjøvik school board and the Dovre Line supervisory committee.
In business and industry, he was a board member of Gjøvik Støperi & Mekaniske Verksted from 1927, Hunton Bruk from 1932 and Nordgaard & Co from 1933.