He finished accounting training in 1904 and worked as a painter in Drammen until 1908 when he travelled to the United States.
In the US he worked as a painter, lumberjack and a construction worker,[1] and then became editor of the socialist publication Social-Demokraten.
[2] Knudsen returned to Drammen in 1920 and worked as a painter foreman until 1923, before he then got a job as an editor in the newspaper Fremtiden.
[3] After a break-in at Knudsen's house by a Norwegian fascist Nasjonal Samling militia led by Vidkun Quisling, on 5 August 1936 and the Trial of the Sixteen Moscow show trial later that month, the Norwegian Minister of Justice, Trygve Lie, eventually ordered on 2 September 1936 that Trotsky and Sedova be evicted from Knudsen's house to a farm at Hurum, where Trotsky and Sedova were placed under house arrest and kept indoors for 22 hours per day, due to the mounting diplomatic pressure put on the Norwegian authorities by the Joseph Stalin led Soviet government.
[1] Trotsky and Sedova were deported from Norway on 19 December 1936, when they were placed on the Norwegian oil tanker Ruth which arrived in Mexico on 9 January 1937.