The son of a farmer, Dahl studied at the universities of Leipzig, Freiburg, Berlin and Kiel.

He became a Privatdozent in 1887; with a habilitation thesis "Ueber die Cytheriden der westlichen Ostsee".

Around this time he traveled to the Baltic states and (1896–1897) to the Bismarck Archipelago near New Guinea.

[1] Dahl remained in Berlin until he retired, and his type collection is held in that museum.

On 19 June 1899 he married Maria Dahl (1872–1972), a co-worker at the zoological institute of Kiel.