Wirtanen entered parliament on the Social Democratic Party of Finland (SDP) list at the 1936 election.

An opponent of fascism, in 1938 in response to the events of Kristallnacht, he likened Nazism's antisemitism with the Finnish far-right's hatred of the Swedish-speaking population of Finland.

Wirtanen was also active in the Socialist Unity Party (SYP) and served as its chairman from 1948 to 1955.

In the 1940's he was a close friend and lover of the author Tove Jansson and a model for the Snufkin character, whose green hat is borrowed from Wirtanen.

The series had to stop earlier than planned, as readers criticized it and especially the Moominpappa character for his bourgeois sympathies.