Seppänen began politics by joining the Social Democratic Youth. In 1974 he joined the Communist Party of Finland. He belonged to the more Eurocommunist majority of the party (as opposed to Taistoist Neo-Stalinist minority). In 1989 Seppänen opposed merging the Communist Party into the new Left Alliance (Vasemmistoliitto), criticizing the procedure as undemocratic. Nevertheless, Esko Seppänen worked in the leadership of the new party until 2001.