Shemariah had a critical mind, and knew Italian, Latin, and Greek. Up to 1305 he studied the Bible exclusively; then he took up Talmudic aggadah and philosophy. His reputation was such that he was called to the court of King Robert of Naples, where he devoted himself chiefly to Biblical studies and wrote commentaries on Scripture. By 1328 he had completed philosophic commentaries on the Pentateuch (especially the story of the Creation), the Book of Job, and Canticles. He aimed at bringing about a union between Karaites and Rabbinites; the Karaites, in fact, recognized and honored him.
The death of a son (1330) interrupted his work for a time, but he soon took it up again. In 1346 he wrote his Sefer ha-Mora, a refutation of the philosophical views on the Creation. Believing that he had placed Rabbinism on a sure foundation, Shemariah undertook, in 1352, a journey to Castile and Andalusia, in order to convert the Karaites. He is said to have pretended to be the Jewish Messiah, and was reviled to such an extent that the government arrested him. He died in prison. Like most of his contemporaries, he was scientifically an epigone of the great philosophers and exegetes. He also wrote Elef ha-Magen (a commentary on the aggadah in the treatise Megillah), some piyyuṭim, and poems.