He was from Ancyra in Galatia, son of a pagan father and a Christian mother named Euphrosyne. His mother prophesied on her deathbed that he would suffer great torments for Christ over many years. After her death he was adopted and reared by a pious woman named Sophia. From the age of twelve, he began to fast and pray like the monks, so that he was soon ordained a deacon, and became Bishop of Ancyra at the age of twenty. His piety and zeal for the faith attracted the attention of the Imperial Governor of the region, who had him arrested. Thus began Clement's twenty-eight years of almost continuous suffering for the Faith. When he stood firm despite many tortures, he was sent to the Emperor Diocletian in Rome. The Emperor showed him a table set with costly vessels on one side, and another decked with instruments of torture on the other, and bade Clement to make his choice. The Saint replied: "These precious vessels remind how much more glorious must be the eternal good things of Paradise; and these instruments of torture remind me of the everlasting punishments of hell that await those who deny the Lord."

The Saint was viciously tortured, then transported to Nicomedia, where a converted pagan named Agathangelus ('good angel') became his companion. For many years they endured unspeakable torments alternating with long imprisonments, but nothing would move them to deny the precious Faith of Christ. After twenty-eight years of suffering, Agathangelus was beheaded; but Clement was briefly paroled and allowed to celebrate the services of Theophany and to give the holy Communion to his fellow-Christians. A few days later, as he was again celebrating the Divine Liturgy, some pagan soldiers burst into the church and beheaded him at the altar.