On December 5, 1925, he tonsured a riassophore monk (similar to temporary vows) at Sretensky Monastery in Moscow.

On October 4, 1927, at the age of seventeen, he took eternal monastic vows with the name Pimen in honor of St. Poemen the Great in the Hermitage of the Holy Paraclete, a skete of the Trinity-Sergius Lavra.

On August 12, 1949, he was appointed namestnik (alderman) of the Pskov-Caves Monastery; on April 13, 1950, he was elevated to the rank of Archimandrite by Metropolitan Gregory (Chukov) of Leningrad.

Because 1970 was the centennial of Lenin's birth, Soviet authorities did not want a church council to select a new Patriarch during that year.

On June 2, 1971, the final day of the Council, Metropolitan Pimen was elected Patriarch of Moscow and All Russia.

Pimen's task was to lead a Christian church in a state ruled by an officially atheist Communist party.

In his post, he worked closely with the Communist authorities, participating in numerous "peace movement" conferences sponsored by the government.

Near the end of his difficult term as head of the Russian Orthodox Church, he organized the celebration of the 1000th anniversary of the Christianization of Rus' in 1988.