Consecrated as Pope Clement III in Rome in March 1084, he commanded a significant following in Rome and elsewhere, especially during the first half of his pontificate, and reigned in opposition to four successive popes in the anti-imperial line: Gregory VII, Victor III, Urban II, and Paschal II. After his death and burial at Civita Castellana in 1100 he was celebrated locally as a miracle-working saint, but Paschal II and the anti-imperial party soon subjected him to a thorough deletio and damnatio memoriae, which included the exhuming and dumping of his remains in the Tiber. He is considered an anti-pope by the Roman Catholic Church.
He was born into the noble family of the Correggio, probably between 1020 and 1030. He had family connections to the Margraves of Canossa. A cleric, he was appointed to the Imperial chancellorship for Italy by the Empress Agnes in 1058, which position he held until 1063. In 1058 he participated in the election of Pope Nicholas II but on his death in 1061, he sided with the philo-imperial party to elect Cadalous of Parma as Antipope Honorius II against Pope Alexander II. However, owing to the campaigns of Godfrey III, Duke of Lower Lorraine, Archbishop Anno of Cologne, and St. Peter Damian, the Church by-and-large rejected Honorius II and acknowledged Alexander II; probably as a result of these activities, the Empress Agnes dismissed Guibert from the Imperial Chancellorship of Italy.
Guibert apparently continued to cultivate his contacts within the German court, for in 1072, Emperor Henry IV named him Archbishop of the vacant see of Ravenna. And, although Pope Alexander II was reluctant to confirm this appointment, he was persuaded by Hildebrand to do so, perhaps as a compromise for peace. Guibert then took an oath of allegiance to the pope and his successors and was installed at Ravenna in 1073.
Shortly after Pope Alexander II died Hildebrand was proclaimed the next pope, being installed as Pope Gregory VII on 29 April 1073. Guibert attended the first Lenten Synods of Pope Gregory in March 1074 in Rome at which important laws were passed against simony and the incontinence of the clergy, but he soon emerged as one of the most visible leaders of opposition to the Gregorian reforms. Having attended Gregory's first Lenten Synods, Guibert refused to attend the next, the Lenten Synod of 1075, although he was bound by oath to obey the summons to attend.
Guibert was unsympathetic to Gregory's opposition to the Imperial Court, which Guibert had served as Chancellor of Italy. By his absence Guibert demonstrated his opposition to Gregory VII, who now suspended him for his refusal to attend the synod.
It was in this same year that Emperor Henry IV began his open war on Gregory. At the synod of Worms in January 1076, a resolution was adopted deposing Gregory, and in this decision the pro-imperial bishops of Transpadine Italy joined. Among these must have been Guibert, for he shared in the sentence of excommunication and interdiction which Gregory VII pronounced against the guilty Transpadine bishops at the Lenten Synod of 1076.
Shortly after, in April 1076, bishops and abbots of the philo-imperial Transpadine party convened at Pavia under the presidency of Guibert and proclaimed the excommunication of Gregory VII; a messenger, bearing a caustic personal letter from Henry, was dispatched with the Pavian reply to the pope. In response, Gregory resorted to still stronger measures with regard to Guibert; he excommunicated Guibert by name at the Lenten Synod of February 1078 and with him his main accomplice Archbishop Tebaldo of Milan.
In response to the action of Henry's 1076 Synod of Worms against Gregory, Gregory excommunicated Henry IV.
During the next four years, the Emperor and the Pope reconciled but then quarreled again, and, facing a rebellion among the German nobles, Emperor Henry threatened to depose Pope Gregory. Carrying out his threats, Henry summoned his German and Transpadine partisans to a Synod at Brixen in June, 1080, which drew up a new decree purporting to depose Pope Gregory VII, and which Henry himself also signed, and then proceeded to elect Guibert, the excommunicated Archbishop of Ravenna, as pope in opposition to Pope Gregory, whom the Synod considered deposed; Guibert took the name Clement III. Henry recognized Guibert as pope, swearing that he would lead him to Rome, and there receive from his hands the imperial crown.
With Rudolph of Swabia, leader of the rebellious nobles, having fallen mortally wounded at the Battle of Mersburg in 1080, Henry could concentrate all his forces against Gregory. In 1081, he marched on Rome, but failed to force his way into the city, which he finally accomplished only in 1084.
Gregory took refuge in Castel Sant'Angelo, and refused to entertain Henry's overtures, although the latter promised to hand over Guibert as a prisoner if the Pope would consent to crown him Emperor.
Gregory, however, insisted as a necessary preliminary that Henry should appear before a council and do penance. The Emperor, while pretending to submit to these terms, tried hard to prevent the meeting of the bishops. A small number however assembled, and, in accordance with their wishes, Gregory again excommunicated Henry.
The latter on receipt of this news again entered Rome on 21 March 1084, and succeeded in gaining possession of the greater part of the city and besieged the Pope in the Castle of Sant' Angelo, while, on 24 March, Guibert was enthroned as pope in the church of St. John Lateran as Clement III, and on 31 March Guibert crowned Henry IV as Emperor at St. Peter's.
However, when the news was brought that Gregory's Norman ally, Robert Guiscard, Duke of Apulia and Calabria, was hastening to his aid, Henry fled Rome with Guibert and, in revenge for Matilda of Tuscany's staunch support for Gregory and the reform party, ravaged her possessions in Tuscany.
Pope Gregory was liberated, but the people were incensed by the excesses of his Norman allies, and he was compelled to leave Rome. Disappointed and sorrowing he withdrew to Monte Cassino, and later to the castle of Salerno by the sea, in 1084, where he died in the following year, 25 May 1085.
Three days before his death he withdrew all the censures of excommunication that he had pronounced, except those against the two chief offenders Henry and Guibert. His last words were:
I have loved justice and hated iniquity, therefore I die in exile.
The German episcopate stood divided. While bishops of Gregory VII's party held a Synod in Quedlinburg, at which they denounced and condemned Guibert, partisans of Henry held a rival Synod at Mainz in 1085, where they approved the deposition of Gregory and the elevation of Guibert.
This conflict continued even after the death of Gregory, during the entire reigns of whose successors, Pope Victor III, Pope Urban II, and Pope Paschal II, Guibert continued to be regarded as pope by Henry and his party.
Victor III, who was elected after a prolonged vacancy caused by the critical position of the Church in Rome, was compelled, eight days after his coronation in St. Peter's on 3 May 1087, to flee Rome before the partisans of Guibert. The latter were in turn assailed by the troops of Countess Matilda, and entrenched themselves in the Pantheon.
The succeeding pope, Urban II (1088–1099), spent most of the first half of his pontificate in exile, in southern Italy and in France. Late in 1093 he managed to obtain a foothold in Rome, with help from the Frangipane family, and gradually expanded his power there.
In June, 1089, at a Synod held in Rome, Clement III declared invalid the decree of excommunication launched against Henry, and various charges were made against the supporters of Urban II, the pope of the anti-imperial party.
Still, the years which followed brought to Urban ever-increasing prestige, while Henry IV's power and influence were more and more on the wane.
The greater part of the city of Rome was captured by an army under Count Hugh of Vermandois, brother of the King of France. The party of Guibert retained only the Castle of Sant' Angelo, and even this in 1098 fell into the hands of Vermandois.
Guibert's influence, after Henry IV's withdrawal from Italy, was largely concentrated in Ravenna and a few other districts of Northern Italy, but he also retained some support in Rome.
In 1099, he repaired to Albano after the accession of Paschal II (1099–1118), hoping again to become master of Rome, but he was compelled to withdraw. He reached Civita Castellana, where he died 8 September 1100. His followers elected a successor to Guibert, the Antipope Theodoric, who, however, was not a serious threat to the popes of the anti-imperial line, now considered canonical.
The elevation of Guibert has to be seen in the wider context of the time: there had been several papal schisms in the recent past, there were political struggles within the Empire, which included the controversy over episcopal investitures.