A northern lumberman's son, Kekkonen studied at the University of Helsinki, receiving bachelor's and doctoral degrees in civil law in 1928 and 1936, respectively. While working as an attorney, he became increasingly involved, from the 1920s, in the agrarian movement and was a civil servant in the Ministry of Agriculture (1927–32). He was a member of Parliament (1936–56) as a representative of the Agrarian Party (later renamed the Centre Party). Originally, he took a hard line against the Soviet Union and, in 1940, was one of only two members of Parliament to vote against ceding any Finnish territory to the U.S.S.R. By 1943, however, recognizing that Germany would lose the war, he concluded that Finland must adopt a policy of friendly neutrality toward the Soviet Union if it wanted to retain its independence.

Holding various ministerial and legislative posts, Kekkonen became prime minister in 1950, during the presidency of Juho Kusti Paasikivi (Paasikivi, Juho Kusti), and he secured the presidency upon Paasikivi's death in 1956. Their careful, but friendly foreign policy toward the Soviet Union came to be called the Paasikivi-Kekkonen line. Kekkonen continued to be reelected to the presidency (1962, 1968, 1978). In 1973 parliament extended his third six-year term in office an extra four years. He resigned in 1981 because of failing health.