(Jan. 844–27 Jan 847)
A Roman aristocrat who was orphaned at the the age of 12 and brought up in the Lateran, he had been made acolyte by Leo III, subdeacon by Stephen IV (V) (a close kinsman), priest by Paschal I, and archpriest by Gregory IV. On Gregory's death the Roman populace proclaimed the deacon John pope, seized the Lateran palace, and enthroned him there. The nobility, meeting in the basilica of S. Martino, elected Sergius, elderly and gout-racked but a grandee of their own class, and swiftly crushed all opposition; at Sergius' request John's life was spared. Because of the tense situation, but also as a gesture of independence, Sergius' consecration was then rushed through without awaiting ratification by the Frankish court.
Emperor Lothair I (840–55) reacted angrily to this flouting of the Roman constitution of 824. In June his son Louis, recently installed at Pavia as viceroy of Italy, with Archbishop Drogo of Metz (801–55), leading Frankish churchman, as his mentor, marched south with a punitive army which, as evidence of the royal displeasure, mercilessly pillaged the papal territories through which it passed. Although Sergius cooled the atmosphere by receiving Louis with ceremonial deference, he had to submit to a tough and protracted investigation of his title by a synod (in which some twenty Italian bishops participated) in St Peter's. Eventually his election was ratified; but in return he, with the citizens of Rome, had to swear allegiance to Lothair and accept that a pope-elect could not be consecrated save on the emperor's orders and in the presence of his representative. Sergius then (15 June 844) crowned young Louis king of the Lombards, anointing him and girding him with a sword. Although declining to swear allegiance to him too (that would have implied that the papal state belonged to his kingdom), he felt obliged to gratify Lothair by nominating Drogo apostolic vicar for the countries north of the Alps. He would not accede, however, to Drogo's proposal that Ebbo and Bartholomew, deposed as archbishops of Reims and Narbonne in 835 for their part in the humiliation of Louis I the Pious (814–40), should be rehabilitated.
Sergius' general administration came in for sharp criticism. An ambitious builder (he enlarged St John Lateran and restored the Marcian aqueduct), he resorted to dubious methods of raising the necessary funds, and incapacitated by his age and gout allowed himself to be dominated by his unscrupulous, power-hungry brother Benedict, whom he made bishop of Albano and who by bribery got himself appointed imperial representative in Rome. Under them simony flourished, and bishoprics and other church offices were sold to the highest bidder. These internal disorders were compounded in Aug. 846 when, in spite of advance warning, Muslim pirates landed in force at the mouth of the Tiber, stormed Porto and Ostia with its fortress Gregoriopolis (the garrison fled), and plundered St Peter's and St Paul's (both outside the Aurelian walls), stripping them of all their treasures. Even the tomb of St Peter was vandalized. Contemporaries were sure that this disaster had been unleashed by Providence as a punishment for the abuses rampant in Rome. The raiders were driven out by the duke of Spoleto. It was little consolation that the Saracen fleet sank on the way home. Sergius himself died suddenly when trying to mediate in a dispute between Patriarchs Venerius of Grado and Andrew of Aquileia.