Monastic life had declined to a low ebb in England in the ninth century, partly due to the ravages caused by Viking attacks, and partly because of a preference for secular clergy, who were cheaper and were thought to better serve the spiritual needs of the laity. Kings from Alfred the Great onwards took an interest in the Benedictine rule, but it was only in the middle of the tenth century that kings became ready to commit substantial funds to its support. Æthelwold became the leading propagandist for the monastic reform movement, although he made enemies by his ruthless methods, and he was more extreme in his opposition to secular clergy than his fellow reformers, Saint Dunstan and Oswald of Worcester. He is nevertheless recognised as a key figure in the reform movement, who also made a major contribution to the revival of learning and the arts. He was an important political figure, backing Ethelred the Unready against Edward the Martyr, and playing a major advisory role during Ethelred's minority.
Æthelwold was born to noble parents in Winchester. From the late 920s he served in a secular capacity at the court of King Athelstan, and according to Æthelwold's biographer, Wulfstan, "he spent a long time in the royal burh there as the king's inseparable companion, learning much from the king's witan that was useful and profitable to him". The king arranged for him to be ordained a priest by Ælfheah the Bald, Bishop of Winchester, on the same day as Saint Dunstan. After a period in the late 930s studying under Ælfheah at Winchester, Æthelwold moved to Glastonbury Abbey, where Dunstan had been made abbot. Here Æthelwold studied grammar, metrics and patristics, subsequently being made dean. During the reign of King Eadred (946-955), Æthelwold wished to travel to Europe to learn more about the monastic life, but Eadred refused permission, and instead appointed him abbot of the former monastic site of Abingdon, Oxfordshire, which was then served by secular priests. The years he spent in Abingdon were extremely productive, and he undertook the building of a church, the rebuilding of the cloister and the establishment at Abingdon of the Benedictine Rule.
When Eadred died, he was succeeded by his nephew, Eadwig, who drove Eadred's chief advisor, Dunstan, into exile. However, Æthelwold attended Eadwig's court in at least some of the years of his reign, 955-59. The future King Edgar had been taught from boyhood by Æthelwold, who evidently inspired his pupil to take an interest in the rule of Saint Benedict. When Eadwig died, Æthelwold naturally backed Edgar's succession. He seems to have been in the personal service of King Edgar in 960-963, as he wrote many of the charters of this period.
On 29 November 963, Æthelwold was consecrated Bishop of Winchester, and the following year, with the connivance of King Edgar and the support of an armed force led by a royal official, he had the clerics of the Winchester Old and New Minsters expelled and replaced by monks from Abingdon. The king had sought the permission of the pope for the expulsion the previous autumn. Between 964 and 971, Æthelwold refounded monasteries at Chertsey, Milton Abbas, Peterborough, Ely and Thorney, and the Nunnaminster nunnery in Winchester. He was also zealous in recovering land which he believed had once belonged to religious communities and subsequently been alienated, and if necessary charters were forged to prove claims to title.
Æthelwold was the principal advocate for the Benedictines during Edgar's reign, the author of all the major works of propaganda produced in England. He had the strong support of Edgar and his wife, Ælfthryth, and his works emphasise the role of Edgar, who he saw as Christ's representative, in restoring the monasteries. He envisaged a major role for Edgar in supervising monasteries, and for Queen Ælthryth supervising Benedictine nunneries. However, he was more extreme in his espousal of monasticism than Dunstan and Oswald, the other great leaders of monasticism in the reign of King Edgar. They followed continental practice in maintaining both monks and secular priests in their households, and did not follow Æthelwold in his dramatic expulsions of secular clerks and replacement by monks. Æthelwold links the terms 'filth' and 'clergy' several times in his writings, regarding them (like other Benedictines) as impure and unfit to serve altars or engage in any form of divine service, because many of them were married and they did not follow a monastic rule.