[1][2] Nilssen was an active socialist already in the 1880s, and was involved in the early labour movement as a founder of worker's associations (Norwegian: arbeiderforeninger).
He was the Labour Party's candidate in the 1906 Norwegian parliamentary election in the single-member urban constituency Lillehammer, Hamar, Gjøvik og Kongsvinger.
[4] In the 1909 Norwegian parliamentary election Nilssen was again selected as the party's candidate, and again faced Thallaug.
Nilssen won a majority in the sub-constituency Hamar, because the conservative vote was split between Thallaug and Gabriel Andreas Stoud Platou.
Platou only stood in his hometown Hamar, and Thallaug carried the three other cities.