He was the third son of Ethelwulf of Wessex and Osburga. She was the daughter of Oslac, Athelwulf's butler. He got his first taste of kingship in 855 when he was left in charge of Kent while his father, King Athelwulf of Wessex, was in Rome. His brother Athelbald was left in charge of the West Saxon kingdom. When their father returned from Rome, Athelbald would not give up the throne. To avoid a civil war, their father Athelwulf took the subkingdom of Kent and Athelbald ruled as king of Wessex. When Athelwulf died in 858, Athelbert was once again appointed king of Kent.