Born in Mosul, he became monk and abbot of the Monastery of Rabban Hormizd near Alqosh. In 1552, a synod of E.-Syr. bishops elected Yoḥannan patr. of the Ch. of E. in order to protest against the not canonical, but in this period institutionalized system of hereditary succession, where the dignity of patr. was mostly passed from uncle to nephew and thus remained within the same family. This synod was not recognized by the legitimate Patr. Shemʿon VII bar Mama and a part of the E.-Syr. bishops. In order to obtain recognition, at the initiative of Latin missionaries, Yoḥannan Sullaqa contacted Roman Catholic authorities and traveled to Rome by way of Jerusalem, where his travel companions sent a letter to Pope Julius III requesting him to ordain Yoḥannan Sullaqa as their catholicos patr. On the basis of the later oral testimony of Yoḥannan Sullaqa in Rome, the Roman authorities incorrectly assumed that Patr. Shemʿon VII had died in 1551, which paved the way to ordain him a bp. and appoint him as ‘Patr. of Mosul’ (1553). After his ordination Yoḥannan Sullaqa took the name of Shemʿon VIII, returned to the Middle East, and, recognized by the Ottoman authorities, established himself in Amid (Diyarbakır), where he ordained a Metropolitan and several bishops in order to secure his succession as head of the new E.-Syr. Uniate Church. He died in 1555, probably assassinated at the instigation of Shemʿon VII (d. 1558), as appears from a panegyric on Yoḥannan Sullaqa, composed by his successor ʿAbdishoʿ of Gazarta.

Yoḥannan Sullaqa is the author of three different professions of faith (1553), originally composed in Syriac. Two of them (of Catholic inspiration) were translated into Latin by Andreas Masius; the third is found in the panegyric of ʿAbdishoʿ and clearly belongs to the Ch. of E.