The patriarch was born Aleksei Mikhailovich Ridiger on 23 February 1929 in Tallinn, Estonia. He entered seminary in Leningrad in 1947 and in 1950 was assigned to his first parish in Estonia.

According to "Religion in the Soviet Union: An Archival Reader," Aleksii was recruited by the Estonian KGB in February 1958 and served for the agency for the next three decades. However, Aleksii's official biography makes no mention of service in the Soviet Union's intelligence service.

In 1961, Aleksii was named bishop (episcop) of Tallinn and Estonia.

In 1964, he was named a permanent member of the Holy Synod of the Russian Orthodox Church and named to a position at the administration of the Moscow Patriarchate.

From 1963 to 1979 he was a member of the Commission for the Holy Synod on Christian unity and interchurch relations.

In 1986, he was named metropolitan for Leningrad and Novgorod.

In 1990, Aleksii was named patriarch for Moscow and all Russia.