He attended the Pontifical Urban University in Rome, where he was consecrated as a priest on December 24, 1923.

In 1924, he returned to Nagasaki, where he was appointed priest of the Tainoura Church in Shin-Kamigotō in the Gotō Islands.

In August 1943, Yamaguchi was sent by the Civil Administration Office of the Imperial Japanese Navy’s Southwest Area Fleet to the Japanese-occupied island of Flores in the Netherlands East Indies.

After the end of the war, Yamaguchi returned to the ruins of atomic-bombed Nagasaki, where he devoted his efforts towards the reconstruction of churches destroyed by the bombing.

On June 8, 1962, he presided[1] over a mass commemorating the centenary of canonization of the Twenty-six Martyrs of Japan.