Paulsen was a keeper at the Botanical Museum of the University of Copenhagen from 1905 to 1920, when he became professor of botany at the Pharmaceutical College in Copenhagen, a position he held until 1947. He studied the flora of Denmark, plankton of the North Atlantic and the flora of Central Asia. He went on expeditions to Northern Persia and Pamir as early as 1898-1899. During his travels through [[Pamir Mountains|Pamir]], he was accompanied by the Danish explorer [[Ole Olufsen]] (1865-1929).