Nils Bergslien was known for his illustrations of folk life, his puck pictures, and for motives from legends and fairytales. His series Nissealbum was published in 1886. He decorated several hotel rooms and churches. He made caricature illustrations and designed the title vignette of the satirical magazine Vikingen. He designed a memorial of fiddler Ola Mosafinn, the bronze relief Ridande Vossabrudlaup.
Over the years Bergslien was given a number of commissions. In 1886, he created the decorations to the tourist ship Britannia; from 1899 to 1900 he decorated several rooms in the Hotel Café Boulevard Norway and Bergen (destroyed by fire 1916) and in the Tyssedal Hotel, and later he made the national romantic decorations for the dining room in the Hotel Vøringfoss in Eidfjord.