While his father, Æthelwulf, was on pilgrimage to Rome in 855, Æthelbald plotted with the Bishop of Sherbourne and the ealdorman of Somerset against him. The specific details of the plot are unknown, but upon his return from Rome, Æthelwulf found his direct authority limited to the sub-kingdom of Kent, while Æthelbald controlled Wessex.

Æthelwulf died in 858, and full control passed to Æthelbald. Perhaps Æthelbald's premature power grab was occasioned by impatience, or greed, or lack of confidence in his father's succession plans. Whatever the case, he did not live long to enjoy it. He died in 860, passing the throne to his brother, Æthelbert, just as Æthelwulf had planned.