The date and place of Pelagius' birth are unknown. The Liber testamentorum includes a genealogy that suggests that Pelagius may have been related to the western Asturian families that founded the monasteris of Coria and Lapedo. He also made a donation to his own canons of properties he owned in Villamoros and Trobajuelo, near León, suggesting perhaps a Leonese connexion.
The earliest known reference to Pelagius is as a deacon at Oviedo in 1096. He was an archdeacon there in 1097. His consecration as the auxiliary of bishop Martin I took place on 29 December 1098. He succeeded Martin four years later, as the choice of Alfonso VI, and with vigour took up the defence of his church's properties and jurisdictions. The Archbishop of Toledo (1086), Bernard de Sedirac, sought to incorporate the sees of Oviedo, León, and Palencia into his province as suffragans. In 1099 Pope Urban II gave the order. In 1104, Pelagius of Oviedo and Peter of León went to Rome to plead their case to the new pope, Paschal II, who granted them a privilege of exemption and made them dependent directly on Rome (1105). At the same time (1104), Pelagius engaged in lawsuits with the count Fernando Díaz, the countess Enderquina Muñoz, and the abbot of Corias to maintain his rights of seignory within Asturias. He was also involved in jurisdictional battles with the neighbouring sees of Burgos (over Asturias de Santillana) and Lugo, and between 1109 and 1113 had to fight off the metropolitan claims of the Archdiocese of Braga as well. In 1121 the Archdiocese of Toledo successfully petitioned Pope Callistus II to remove Paschal's 1105 exemption, though this was regained in 1122.
Pelagius was generally on good terms with Alfonso VI (died 1109) and his successor, Urraca (died 1126). After 1106 no new Count of Asturias was appointed and it seems that the title lapsed, while a castellan, a novus homo, with lesser authority replaced the last count. This was probably in the interests of Pelagius and his authority, since the county of Asturias corresponded to the centre of his diocese. The bishop gave Urraca political support against both her husband, Alfonso the Battler of Aragon, and her son, the future Alfonso VII, who was in conflict with his mother after 1110. She in turn made grants to Oviedo on three separate occasions, in 1112, 1118, and 1120 and Pelagius was the dominant Asturian at court, confirming fifteen royal charters during her reign. Pelagius had a part in reconciling the queen and her son at a council of the realm in Sahagún (1116). After Alfonso's accession he never recovered his importance, rarely appearing at the new king's court and never receiving a gift from him.
In 1130 Pelagius was deposed by a synod held under Cardinal Humbert at Carrión, along with Diego and Munio, bishops of León and Salamanca, and the abbot of Samos, because they had opposed the marriage of Alfonso VII and Berenguela of Barcelona (1127) on grounds of consanguinity. Their deposition was politically motivated, engineered by Alfonso and the prelate Diego Gelmírez. During the last decades of the eleventh century and the first of the twelfth, Santiago de Compostela became one of the leading centres of pilgrimage in the among the Catholic faithful, aided by the efforts of its archbishop, Diego Gelmírez. The rivalry between Pelagius and Diego can be seen in the former's attempt to establish Oviedo as a comparable destination for pilgrims, by expanding the cult of the relics of the Cathedral of San Salvador, most importantly the alleged Sudarium of Christ. He has even been credited with the creation of the Arca Santa to house his cathedral's relics.
Pelagius continued to live in Oviedo and be addressed as bishop. When his successor, Alfonso, died in January 1142, Pelagius took up the diocesan administration again until early in the summer of 1143. By June the see was being administered by Froila Garcés, the archdeacon, and in September Martin II was elected bishop at a council in Valladolid. He had planned his own funeral and had reserved a space in the Cathedral of San Salvador for his burial. Nevertheless, his death came unexpectedly while he was visiting Santillana del Mar, and there he was buried.