It was founded in 1944 as the anti-communist laws in Finland were repealed due to the demands of the Soviet Union, and lasted until 1990, when it merged into the newly formed Left Alliance.

The SKDL enjoyed its greatest electoral success in the 1958 parliamentary election, when it gained a support of approximately 23 per cent and a representation of 50 MPs of 200 total, making it the largest party in the Eduskunta.

The late 1960s governments, led by social democrats and including centre, were called popular front by the SKDL.

The number of communist party members amongst the SKDL MPs constantly increased from 1945 on, even though many prominent left-wing socialists and former social democrats had joined the alliance in the 1940s.

One of the few organized non-SKP forces in SKDL was the Socialist Unity Party (SYP) which was founded mainly by former social democrats in 1946.