The Kingdom of Kent is said to have been founded by Hengist, but the most famous Kentish ruler was Æthelbert, who ruled his kingdom in the late 5th and early 6th Century. Sources say that Æthelbert was descended from Hengist.

Æthelbert is known for being the first Anglo-Saxon king to convert to Christianity. A monk named Augustine (later named St. Augustine) arrived in Canterbury, the capital of Kent, in 597. Augustine, on a mission authorized by the Catholic Pope Gregory I, found a warm welcome from Æthelbert, in large part because the king's wife was already a practicer of the Christian religion.

Æthelbert also had a few significant Christian places of worship built, notably St. Paul's in London and St. Martin's in Canterbury.

What also became commonplace was consolidation. Kent wasn't always just one kingdom. For several years, the area was two kingdoms, East Kent and West Kent; for a time, each area had its own king. Æthelbert proved a unifying force and was king of all the people in Kent for about 25 years.

This unification extended beyond the Kentish borders. The Anglo-Saxon Chronicle says that Æthelbert was at one time bretwalda, a kind of overlord who would have had dominion over more than one Anglo-Saxon kingdom.

Æthelbert is also well-known for the Law of Æthelbert, a 90-section set of laws that, among other things, set out punishments for breaking those laws. Those punishments are arranged according to social rank.

The Law of Æthelbert was written in Old English and, as such, is the first Germanic-language law code. One copy survives, the "Rochester Book," which was copied out only in the 12th Century.

Æthelbert's wife, Bertha, is revered in English and Christian history for her role in Æthelbert's conversion. Bertha was a Frankish princess and was already a Christian when she arrived in Kent, a few years before Augustine arrived.