Mentz studied
botany at the
University of Copenhagen under professor
Eugen Warming. He was employed by the
Danish Moor Reclamation Society ("Det danske Hedeselskab") from 1899 to 1923. In 1912 he defended his
doctoral thesis on the “Current Vegetation of Danish Bogs and Swamps” (Mentz 1912). From 1923 to 1938, he was professor of
botany at
Royal Veterinary and Agricultural University. He edited two voluminous colour plate floras – ”Billeder af Nordens Flora” and “Vilde Planter i Norden”.
Mentz was chairman of the
state advisory board on conservation (”Naturfredningsrådet”) 1925–1944. However, with his background in
moor reclamation, he had a foot in both camps – utilitarianism and conservationism.