He was the son of Eormenric, whom he succeeded as king, according to the Anglo-Saxon Chronicle. Gregory of Tours, who was a close acquaintance of Queen Ingoberg (the mother of Ethelbert's wife Berthe), twice calls him simply "a man of Kent", indicating that he was not king at the time Gregory's History of the Franks was written, and that Ethelbert more likely became king closer to 590.

The Anglo-Saxon Chronicle records that he attempted at one stage to wrest the position of Bretwalda from Ceawlin of Wessex but was unsuccessful. His standing was advanced by his marriage with Berthe, daughter of Charibert, king of the Franks, thus building an alliance with the most powerful state in Europe at that time. This prestige enabled him to claim the title of Bretwalda after Ceawlin's death.

The influence of Berthe, who had brought her chaplain Liuhard (or Letard) with her to Kent, may have led to the invitation to Pope Gregory I to send missionaries from Rome. Augustine arrived in Canterbury in 597, and tradition has it that he baptized Ethelbert only a few days after landing in Kent, although a letter from Gregory to Berthe suggests that it cannot have happened before 601.

Ethelbert was later canonised for his role in restoring Christianity to England.