Æthelfrith appears to us as a great warrior king. Bede describes him as being "very brave and most eager for glory" and says that he ravaged the Britons more thoroughly than any other king. He was established as king of Bernicia by 592. We know nothing of the circumstances of his accession, but one source gives the information that he gave the coastal stronghold of Dinguoaroy to his wife Bebbe and named in Bebbanburh in her honour. The place is now called Bamburgh. It is the place where Ida is said to have established the Bernician kingship in 547. So it looks as though Æthelfrith was making a deliberate gesture to appropriate the prestige of the ancestral tradition to himself and this may have been a strategy to consolidate his position against dynastic rivals. If, as has been suggested, Æthelfrith was the founder of the palace complex of Yeavering, on the edge of the Cheviot hills, we may be seeing here a sign of him developing the institutions of the state.

Secure in his domestic power base, Æthelfrith turned to external matters. He was challenged in 603 by Ædan the king of the Irish realm of Dal Riata centred in Argyll in western Scotland, who raised an army against him. This suggests that Bernician power had extended into what is now southern Scotland, threatening Dal Riatan interests. Æthelfrith won a comprehensive victory in battle at a place Bede calls Dagsestan. He also campaigned against British kingdoms of what is now north-west England.

Aggressive military actions were not necessarily intended to extend the borders of the state directly; but conquest enabled a king such as Æthelfrith to extract payment of tribute from other dependent kings.

In the following year, 604, Æthelfrith did make a move to expand his rule directly by taking over the neighbouring kingdom of Deira. He killed the incumbent king and drove out Edwin his heir, taking the kingship to himself. Then to consolidate his position he married the Deiran princess Acha, Edwin's sister.

The next we hear of Æthelfrith is in 615, campaigning to the south-west of his own lands and engaged in a brutal battle with the Britons by the old Roman fortress town of Chester. This was an area under the influence of the Mercian kingdom of the English midlands. Æthelfrith had a direct and urgent concern here, for Edwin had taken refuge with King Cearl of Mercia and formed an alliance by marrying the princess Cwenburh. So long as Edwin was at large, Æthelfrith's position was potentially insecure and his dynastic plans at risk. For his strategy to absorb the Deiran kingship depended not just on his marriage to Acha, but also on eliminating the males of the Deiran house.

Æthelfrith's moves against the Mercians sent Edwin further away, seeking protection from King Rædwald of East Anglia, the most powerful English king of his day. Æthelfrith could not reach him directly in East Anglia, but he offered bribes to Rædwald to have Edwin killed. Rædwald refused the bribes and 616 he raised an army against Æthelfrith, marching north with Edwin in his retinue. They caught Æthelfrith's army unprepared beside the River Idle, south-east of Doncaster, and he himself was killed. Edwin was installed as king and Oswald and Oswiu, the sons of Acha and Æthelfrith, fled to western Scotland. In the short term, Æthelfrith's dynastic plans had failed; but nineteen years later Oswald would re-gain his father's throne.

Æthelfrith's life is on the margins of recorded history and our view of him, from the scant information available, is one-dimensional: we see him just as a warrior and we have little insight into other aspects of his character or actions. As a warrior, he began the process which would in time result in a fully unified and integrated Northumbrian kingdom from the two smaller realms. In an historical perspective, he stands at the head of a dynasty. Except for reign of King Edwin, for the whole of the 7th century, and beyond at AD 716, Northumbria was ruled by the descendants of Æthelfrith in a line of kings uninterrupted for three generations beyond his own. Northumbria of the 8th century was weakened by the rivalry of competing families. At this early and fluid stage in the formation of states, Æthelfrith's was a uniquely stable and successful dynasty.