He was a theatre director at Den Nationale Scene, at Folketeatret, and for two separate periods at Det Norske Teatret.

His film debut was in Syndere i sommersol from 1934, and he also played in To levende og en død from 1937.

The performance created considerable interest and controversy due to its socially critical content.

It proved to be a financial success partly because of Nilsen's advanced directing and set design.

Together with Sigval Maartmann-Moe, he co-directed the film Dei svarte hestane from 1951, based on Tarjei Vesaas's novel with the same title.