He was born in Lillehammer as a son of shoemaker Mathias Nilssen (1834–1920) and his wife Eline Pedersen (1835–1918).

He was also member of the socialist youth club Friheden in both Kristiania and Sarpsborg (where he lived in 1894).

[1] He joined the Norwegian Labour Party, and became a member of the central board in 1900.

[1] He took part at the founding congress of the Labour and Socialist International in 1923; the other Norwegian delegates were Arne Magnussen, Michael Puntervold and Olav Kringen.

[5] Nilssen also served as Minister of Labour in the two-week Hornsrud's Cabinet between January and February 1928.

Nilssen participated in the Riksråd negotiations in 1940 between Germans and those parliamentarians who had not fled the country, in which the Presidium infamously asked King Haakon VII to abdicate.

[2] After the occupation and war, he was dropped by the Labour Party as their member of the Presidium, and was not summoned to the relevant meetings and forums.