Sergius is the son of Benedictus, and traditionally was believed descended from a noble Roman family, although it has been speculated that he was in fact related to the family of Theophylact, Count of Tusculum.

He had been ordained as a subdeacon by Pope Marinus I, followed by his being raised to the deaconate by Pope Stephen V. During the pontificate of Pope Formosus (891–896), he had been a member of the party of nobles who supported the Emperor Lambert, who was the opponent of Formosus and the pope’s preferred imperial candidate, Arnulf of Carinthia.

Formosus had consecrated Sergius as bishop of Caere (Cerveteri) in 893, apparently in order to remove him from Rome.

Sergius had ceased to act as bishop of Caere with the death of Formosus in 896, as all of the ordinations conferred by Formosus were declared null and void although Formosus’ ordination of Sergius was later reconfirmed by Theodore II.

He had also actively participated in the farcical Cadaver synod that condemned the pontificate of Formosus.

With the death of Theodore in 898, Sergius, with a small following of Roman nobility led by his father Benedictus, had attempted to have himself elected pope, contrary to the wishes of the emperor Lambert, who was also duke of Spoleto.

Although Sergius was actually elected, a rival candidate, Pope John IX (898–900), was also elected With Lambert’s support, John had been successfully installed as pope, and one of his first acts had been to convene a synod which excommunicated Sergius and his followers.

Sergius was then forcibly exiled by Lambert, fleeing to his see at Caere, where he had placed himself under the protection of Adalbert II, Margrave of Tuscany.

By the time the Antipope Christopher (903–904) seized the chair of Saint Peter by force, circumstances had changed at Rome, with the rise of the magister militum Theophylact, Count of Tusculum, who had been stationed at Rome by the retreating emperor Louis the Blind in 902.

Putting himself at the head of a faction of the nobility, Theophylact had revolted against Christopher, and asked Sergius to return to Rome to become pope.

Sergius accepts, and with the armed backing of Adalbert II, he enters Rome, by which stage Christopher has already been cast into prison by Theophylact.

Sergius is then consecrated Pope on January 29, 904.