Ælfheah was born around 953,[8] supposedly in Weston on the outskirts of Bath,[9] and became a monk early in life.[10] He first entered the monastery of Deerhurst, but then moved to Bath, where he became an anchorite.[11] He was noted for his piety and austerity and rose to become abbot of Bath Abbey.[10] The 12th century chronicler, William of Malmesbury recorded that Ælfheah was a monk and prior at Glastonbury Abbey,[12] but this is not accepted by all historians.[10] Indications are that Ælfheah became abbot at Bath by 982, perhaps as early as around 977. He perhaps shared authority with his predecessor Æscwig after 968.[12]

Probably due to the influence of Dunstan, the Archbishop of Canterbury (959–988), Ælfheah was elected Bishop of Winchester in 984,[13][14] and was consecrated on 19 October that year.[11] While bishop he was largely responsible for the construction of a large organ in the cathedral, audible from over a mile (1600 m) away and said to require more than 24 men to operate. He also built and enlarged the city's churches,[15] and promoted the cult of Swithun and his own predecessor, Æthelwold of Winchester.[11] One act promoting Æthelwold's cult was the translation of Æthelwold's body to a new tomb in the cathedral at Winchester, which Ælfheah presided over on 10 September 996.[16]

Following a Viking raid in 994, a peace treaty was agreed with one of the raiders, Olaf Tryggvason. Besides receiving danegeld, Olaf converted to Christianity[17] and undertook never to raid or fight the English again.[18] Ælfheah may have played a part in the treaty negotiations, and it is certain that he confirmed Olaf in his new faith.[11]

In 1006, Ælfheah succeeded Ælfric as Archbishop of Canterbury,[19][20] taking Swithun's head with him as a relic for the new location.[11] He went to Rome in 1007 to receive his pallium—symbol of his status as an archbishop—from Pope John XVIII, but was robbed during his journey.[21] While at Canterbury, he promoted the cult of Dunstan,[11] ordering the writing of the second Life of Dunstan, which Adelard of Ghent composed between 1006 and 1011.[22] He also introduced new practices into the liturgy, and was instrumental in the Witenagemot's recognition of Wulfsige of Sherborne as a saint in about 1012.[23]

Ælfheah sent Ælfric of Eynsham to Cerne Abbey to take charge of its monastic school.[24] He was present at the council of May 1008 at which Wulfstan II, Archbishop of York, preached his Sermo Lupi ad Anglos (The Sermon of the Wolf to the English), castigating the English for their moral failings and blaming the latter for the tribulations afflicting the country.[25]

In 1011, the Danes again raided England, and from 8–29 September they laid siege to Canterbury. Aided by the treachery of Ælfmaer, whose life Ælfheah had once saved, the raiders succeeded in sacking the city.[26][c] Ælfheah was taken prisoner and held captive for seven months.[27] Godwine (Bishop of Rochester), Leofrun (abbess of St Mildrith's), and the king's reeve, Ælfweard were captured also, but the abbot of St Augustine's Abbey, Ælfmær, managed to escape.[26] Canterbury Cathedral was plundered and burned by the Danes following Ælfheah's capture.[28]