Kuusinen joined the Social Democratic Party in Finland in 1905. Subsequently he held various important posts in the party, serving as minister of education in the short-lived Finnish socialist regime in early 1918. He fled to Russia in the same year, after the Finnish War of Independence had brought the socialist regime to an end, and became a key organizer of the Finnish Communist Party. Remaining in exile during the interwar years, he occupied the powerful position of secretary of the Comintern.
With the start, in 1939, of the “Winter War” between the U.S.S.R. and Finland, which had been assigned to the Russian sphere of influence in the German-Soviet Nonaggression Pact of 1939, Kuusinen was named head of a puppet Finnish socialist government. When the Soviet Union came to terms with Finland early in 1940, however, his government was quietly dissolved. From 1940 to 1956 he served as president of the supreme soviet (assembly) of the Karelo-Finnish Soviet Socialist Republic, which resulted from the union of Soviet eastern Karelia and Finnish western Karelia at the conclusion of the war in 1940. From 1946 to 1953 and from 1957 until his death, he was secretary and a presidium member of the Central Committee of the Communist Party of the Soviet Union.