Clement was one of the closest disciples of Sts. Cyril and Methodius — present with them in Moravia, in Rome (where Pope Hadrian II ordained him), and at Methodius's deathbed at Velehrad in 885. Driven from Moravia by Frankish persecution, he was sold into slavery by Frankish bishops and finally ransomed by Bulgarian envoys, reaching Boris I of Bulgaria in 886.
Boris settled him at Ohrid (in modern North Macedonia) and entrusted him with the founding of a Slavic literary school. There, around 893, he developed from Cyril's Glagolitic the simpler Cyrillic alphabet — letters drawn from the Greek uncial script with the addition of new characters for Slavic-specific sounds — which would become the writing system of the Bulgarian, Serbian, Russian, Ukrainian, Belarusian, and Macedonian languages.
He was consecrated by Boris's successor Simeon as the first Slavic-speaking bishop ('of Velika') around 893. He governed the school of Ohrid for nearly thirty years, composing hundreds of hymns, sermons, and Lives of the Slavic saints — the foundational literature of medieval Slavic civilization.
He died at Ohrid in 916 and is buried in the great monastery of St. Pantelejmon at Plaošnik in Ohrid, which he founded and which is venerated to this day as the cradle of Slavic Christianity.