A native of Koshang, Marcos traveled with Rabban Bar Sauma, an ascetic Nestorian monk from Mongol-controlled China to Jerusalem, however, due to the war between the Mongols and Mamluks they were prevented to reach the final destination. Patriarch Denha I of the Church of the East recalled them and consecrated Markos as the bishop of Katay and Ong, with the name Mar Yahballaha. However, both of them opted to remain in monasteries in Mosul.