Cadwallon apparently had very long arms. The appendage to his name means "Long Hand" and Iolo Goch explains that he could "reach a stone from the ground to kill a raven, without bending his back, because his arm was as long as his side to the ground." He was the eldest son of King Einion Yrth of Gwynedd and was almost certainly the first generation of the dynasty to be born in Wales. His wife was a native of Nant-Conwy, but her mother, like Cadwallon's own family, was a Northern migrant.
Cadwallon seems to have inherited the western portion of his father's Kingdom around what is now central Gwynedd. Not satisfied with this small area of land, Cadwallon joined forces with his cousins, Princes Cynyr, Meilir and Yneigr of Ysfeilion, to extend the policies of his grandfather, Cunedda Wledig, and continue to wrest the remainder of North Wales (Lleyn, Arfon & most of Ynys Mon (Anglesey)) out of Irish control. Ynys Mon (Anglesey) was the central Irish power-base and, upon his succession to the throne, Cadwallon and his war-bands made a concerted push to evict them. He defeated the invaders in several bloodthirsty clashes including the Battle of Cerrig-y-Gwyddyl, at which the Welshmen tied their feet to their horses, in case their courage should desert them. In AD 517, Cadwallon forced the Irish into a mass retreat back to Holy Island. From here, many of them escaped in boats but their leader, Serigi Wyddel (the Irishman), was cut down at Llam-y-Gwyddyl (Irishman's Leap). His bravery was much respected and the Welsh who later erected a church over his grave at Llanbabo.
The palace of Cadwallon's early years on the throne was at Bodysgollen near Llanrhos, but he later favoured Ynys Mon (Anglesey) and set up the Royal court at Aberffraw, on its west coast. In Arthurian literature Cadwallon appears to have been remembered as King Cradelmant of Northgalis, one of the eleven Kings who rebelled against High-King Arthur at the beginning of his reign.