He was born in Halse as a son of shipmaster and merchant Niels Clemetsen Nielsen (1795–1845) and his wife Andrea Marie Møller (1802–1866).

[1] Important paintings include Hvalørhei (1874), Skovbillede (1896), Morgen ved Ny-Hellesund (1885, one of several from Ny-Hellesund), Ensomt sted (1901), Fra Bankefjorden (1910) and Kveld på Jæren (1925)[1][2] Most of his paintings portrayed Western and Southern Norway,[1] but also Østfold.

[1] Eleven of his works are owned by the National Museum of Art, Architecture and Design.

[1][4] Nielsen was also decorated as a Knight, First Class of the Royal Norwegian Order of St. Olav in 1890.

After a period of grief, he married Laura Tandberg (1857–1928) in February 1888 in Risør.