He is remembered for his paintings of the allotments and outskirts of Copenhagen and later for his landscapes of the Bramsnæs Vig area in northwestern Zealand.
His Spisehuset (1935) depicts the loneliness of a poor city dweller while other works cover the disadvantaged and homeless inhabitants of Christianshavn.
From 1954, Hjorth Nielsen also created a series of works depicting the roofs of the city he observed from his penthouse on Gammel Strand.
His landscapes included summer scenes at Vester Åby on Funen and around Bramsnæs Vig on the west coast of Zealand; the houses and vegetation in the bottom half of the paintings contrasted with the stillness of the low-set horizon.
He was also active in etchings and woodcuts covering studies of models at the Academy, sketches during his travels or landscape depictions.