[1][2] Kosonen first worked as an editor at the Työmies newspaper, which was founded in 1895.
[3] In 1898, he was forced to leave Finland due to the anti-socialist policies of the Finnish government under Russian governor Nikolay Bobrikov.
[4] He was involved in the activities of the Social Democratic Party, and served as editor of the Kansan Lehti newspaper.
[6] During the 1905 Russian revolution, Kosonen organised shelter for refugees from the Baltic region of the Russian Empire, also helped organise the December 1905 Tampere bolshevik conference.
[6][2] In February 1906, he helped Latvian revolutionary Jānis Čoke, who was later revealed to have taken part in the robbery of the Russian State Bank branch in Helsinki.