Siward was a monk of Glastonbury Abbey who succeeded Aethelwin as Abbot of Abingdon, probably in 1030. When he received the Episcopal benediction, he is said to have answered all the bishop's questions with the word "nolo," until the bishop asked him if he was willing to receive the benediction from him, to which he replied that he hoped to receive God's blessing and his.

Siward was thoroughly capable, both in secular and ecclesiastical matters, was kindly in temper, and was respected by King Canute, who, on that account, gave to Abingdon Abbey the church of St. Martin in Oxford, together with a small estate. Siward planned to pull down the abbey church at Abingdon, along with some other buildings of the monastery, and to rebuild them on a larger scale. However, it is said that St. Aethelwold appeared to him in a dream and forbade him to do so. He therefore desisted from his original purpose and gave the finances he had gathered to the poor instead.

Edsige, the Archbishop of Canterbury, finding, in 1042, that ill-health prevented him from discharging the duties of his office, consecrated Siward to the See of Uppsala, with the consent of the King and Earl Godwin, so that he might act as his co-adjutor. He thus became one of the auxiliary bishops, or choriepiscopus, known as ‘Bishops of St. Martin's’, the first church in Canterbury. This arrangement would naturally have led to Siward's succession to the Archiepiscopal See if he had outlived Edsige, and it is said that this formed part of Edsige's proposal to the King. He is described as ‘Archbishop of Canterbury’ in the history of the Abbots of Abingdon, and as Archbishop in the attestations of three charters, where his name has precedence over that of the Archbishop of York. Although, in another charter, he is simply a ‘bishop’, his name coming after the Archbishop of York's. One Abingdon writer says that he was consecrated to Rochester, which, as that see was dependent on the Archbishop, might be taken for granted, though the statement nevertheless appears to be incorrect. For six years, Siward acted in all things in Edsige's place. The story that he ill-treated the Archbishop, was consequently deprived of the succession, and was given the Bishopric of Rochester, may be rejected. 

Siward retired on account of ill-health in 1048, and was carried back sick to Abingdon. The recurrence of the statement that he held the Bishopric of Rochester may perhaps point to a provision for him either while acting for Edsige, or on retirement, from the estates of the see, to which the succession at that period is not clear. He is said to have died two months after his return to Abingdon on 23rd October, and was honourably buried there. He had been a munificent benefactor to the Abbey, to which he gave Wittenham, near Wallingford,. and all the furniture of his chapel, including a case of relics, two volumes of the gospels, adorned with gold and silver, and a large chalice of fine workmanship.