Aethelbald was the second son of Aethelwulf, King of Wessex. We know little about his early life, but by the 840s his name was appearing as a witness to his father's charters. In AD 850 he was raised to the rank of ealdorman. In 855 Aethelwulf went on a pilgrimage to Rome, and Aethelbald was named regent of Wessex in his father's absence.
While abroad on his pilgrimage Aethelwulf married Judith, the 13-year-old daughter of the Carolingian king Charles the Bald. The marriage was a threat to Aethwlbald's hopes of inheriting his father's throne. It seems likely that Aethelbald plotted with the Bishop of Sherborne and Eanwulf, Alderman of Somerset, to prevent the king's return from Europe.
The plot failed, and Aethelwulf returned with his young wife. To prevent civil war Aethelwulf allowed Aethelbald to rule the western part of his realm while he ruled the eastern part, including Kent. Aethelwulf died in 858, but Aethelbald did not take over the entire kingdom; he ruled Wessex while his younger brother Aethelberht ruled Kent.
He incurred the wrath of the Church by marrying his father's widow, Judith. Bishop Asser judged it a move contrary to Christian dignity and even pagan custom. Aethelbald was not swayed however; the lure of a link to the Carolingian dynasty was too strong.
There is little more to say of Aethelbald's short reign, as he died after only four years on the throne. Only one charter from his reign survives, and that was witnessed by both his wife Judith and his brother King Aethelberht of Kent, suggesting that the brothers were on good terms. Aethelbald died in 860 at Sherborne. There is a memorial to him in Sherborne Abbey.
Bishop Asser wrote that his reign was 'two and a half lawless years', and that the king himself was 'iniquitous and grasping'. Asser's assessment was hardly impartial for he objected to the king's marriage to Judith and to his revolt against his father. Aethelbald was a strong and warlike leader, good qualities for the time.
After Aethelbald's death, Judith returned to the continent, where her father sent her to a nunnery, whence she later eloped with the Count of Flanders.