He joined the Swedish Social Democratic Party and in 1916 was elected to the Riksdag. But in 1917 Ström broke with Hjalmar Branting and sided with the far left wing of the party headed by the communists Zeth Höglund and Ture Nerman. This group supported the Russian Bolsheviks and would soon become the (original) Swedish Communist Party.
Fredrik Ström was, together with Ture Nerman and Stockholm's mayor Carl Lindhagen, part of a small delegation of Swedish Communists who greeted Lenin during his short visit in Stockholm in April 1917. The Swedish Communists took Lenin to the PUB department store where they bought him a brand new suit so he would look good returning home to revolutionary Petrograd.
Ström was head of the Stockholm Comintern liaison with Western Europe from 1919 – 1920, and he frequently visited the Soviet Union for meetings including the third congress of the Communist International, held in Moscow 1921.
Ström supported Lenin, Trotsky and the October Revolution, but he disagreed with Stalinism and in 1926 he found his way back to the Swedish Social Democratic Party, although he still considered himself a Communist.