[1] In 1912, during an intermittent stay in Norway, he had become active in Norges Socialdemokratiske Ungdomsforbund, and became acquainted to Martin Tranmæl.

While staying in Germany, he had written articles for the newspaper Ny Tid, where Tranmæl was the editor-in-chief.

The radical wing of Madsen, Tranmæl and others later assumed control over the party at the national convention in 1918.

[1] He was also a secretary in the Norwegian Confederation of Trade Unions, and was known as a supporting figure of chairman Ole O. Lian.

He published several pamphlets in the years around 1920, including 1917's Taylor-systemet: videnskabelig ledelse av industriene, an exploration of scientific management.

[1] In 1923, the Labour Party suffered a split due to disagreements over their membership in the Comintern as well as the Twenty-one Conditions.

[1] In 1931 he left the position as parliamentary leader as he was elected deputy chairman as well as treasurer of the Confederation of Trade Unions.

On 20 March 1935, when the cabinet Nygaardsvold assumed office, Madsen was appointed Norwegian Minister of Trade, Shipping, Industry, Craft and Fisheries.

[2] However, only months before the next election was scheduled, Norway was invaded and subsequently occupied by Germany as a part of World War II, effectively suspending the Parliament.