He was born in Bergen as a son of Albert Henrik Mohn (1811–1894) and Ida Neumann (1814–1864).
[2] He finished his secondary education at Bergen Cathedral School in 1855, and graduated from the Royal Frederick University with the cand.jur.
He was soon hired in the Statistics Office, and together with Anders Nicolai Kiær he led its transition into an independent research directorate, renamed Det statistiske Centralbureau in 1876.
Norway as a country implemented the use of large, quantitative statistics in politics quite early.
Mohn was also involved in the workers' society Christiania Arbeidersamfund, and through his research on child labour among other things, he capacitated social reform.