Very little is known about Æthelbert, who took his rightful place in the line of succession to the throne of Wessex at around 30 years of age.

Like all other rulers of his day, he had to contend with Viking raids on his territories and even had to battle them in his capital city of Winchester. Apparently, his military leadership was adequate, since, on this occasion, the Vikings were cut off on their retreat to the coast and were slaughtered, according to a contemporary source, in a "bloody battle."