Former monk of the Monastery of Eusebona, he became the synkellos (secretary) of Patr. Athanasios I Gamolo , whom he succeeded in 630/1. As patr. he is especially known for his conversations with a Muslim Emir, probably ʿUmayr b. Saʿd b. ʿUbayd, which took place in 644. The Syriac report of this encounter by his secretary is preserved in a single ms. written in 874. There are a number of elements in the text, especially a discussion on inheritance law, which suggest that the original report was reworked by later redactors, possibly in the 8th or even the 9th cent.
According to the Chronicle of Dionysios of Tel Maḥre , preserved in the work of Michael Rabo , Yuḥanon III (Yuḥanon I, if one does not follow the W.-Syr. computation of the patriarchal lists) would have been responsible for the supervision of the translation of the Gospel from Syriac into Arabic (not preserved). He composed a letter to Marutha on the persecution of the Miaphisite Christians in Persia by Barṣawma of Nisibis (preserved in Michael’s Chronicle). His other writings comprise: a pleropuryā (apologetic treatise) on the aphthartodocetic doctrines of Julian of Halicarnassus ; another on Christology with strongly worded condemnations of the ‘Nestorians’ and the Chalcedonians; a homily on the consecration of the Holy Myron (and, according to the Chronicle of Siirt , ch. 110 [PO 13.4, 634]; the text itself of the consecration of the Myron as well as of the Blessing of the Water at Epiphany); a number of sedre, prayers of intercession arranged in a specific order (sedrā) — which gave him his name d-sedre ‘of the sedre’ — for different occasions; and possibly an Anaphora.