The SYP was founded in March 1946 by socialists working inside the communist-dominated Finnish People's Democratic League (SKDL).
The founders of the SYP included the so-called "socialist six" (in Finnish "kuutoset") who were expelled from the SDP parliamentary group in 1940 before the summer 1941 Continuation War against Soviet Union.
In 1955, Wirtanen organized an extraordinary party congress in which it was decided that the SYP would leave SKDL.
The secretary of the congress was Wirtanen's wife and no representatives from the important Southern Workers Organization of Helsinki were present.
Many leading members and sections of the SYP dismissed the congress, left the party and continued their work inside the SKDL.