The most notable feature of his reign (of which little information exists) is the growing strength of the nearby kingdoms of Mercia, under the rule of Pybba and his son Penda, and of Bernicia, under Æthelfrith. Some evidence exists to suggest that Iago may have entered into some sort of alliance with Pybba, trading the latter's promise not to attack Gwynedd in exchange for Gwynedd's support against Northumbria, should that support be needed. In 604, after being driven from his throne by Æthelfrith, Edwin, king of Deira, may have sought refuge at the Iago's court. Perhaps as a result, Æthelfrith turned his wrath against Gwynedd, killing the monks at a monastery at Bangor, and finally facing the forces of Gwynedd and Powys at the Battle of Chester in 613.

It is not known whether or not Iago took part in this battle: he may have done so, perishing amidst the slaughter, or he may have already abdicated his throne in favor of his son, Cadfan, and died later that year in a monastery.