In 864 Formosus was made bishop of Portus. He did missionary work for the Bulgarians who then asked that he be made their bishop. This was forbidden by the Second Council of Nicaea and so Pope Nicholas I denied the request. In 875, he convinced Charles the Bald, who was King of the Franks to be crowned emperor. He may have been a potential candidate for Pope about this time but political complications led to him flee Rome and the court of the Pope John VIII. Pope John VIII then convened a synod and demanded that Formosus return. If he did not he would be excommunicated on a range of charges including aspiring to the Bulgarian Archbishopric and the Holy See, opposing the Emperor and deserting his diocese. In 878, it was decided that Formosus would not be excommunicated as long as he swore never to return to Rome or perform any priestly duties.
The 9th and 10th centuries were a period of rapid Papal successions and with the papacy of Marinus I in 883, Formusus was restored to his position at Portus. Then after the reigns of Pope Hadrian III and Pope Stephen V, Formosus was elected Pope on October 6th, 891. The vote was unanimous.
In 892, he crowned Lambert of Spoleto co-emperor of the Holy Roman Empire, just as Guy III of Spoleto had been crowned by John VIII. But in 893, Formosus was concerned about the aggression that Guy III was showing and worried about the fate of Rome. So he asked the Carolingian Arnulf of Carinthia to invade Italy in order to take the imperial crown. Arnulf agreed and invaded, but it failed. Then Guy III died soon after and in 895 Formosus once against asked Arnulf to take the Imperial crown. This time he succeeded and Formosus crowned him Holy Roman Emperor.
In 896 both Arnulf and Formosus died. Pope Boniface VI took over but died two weeks later. Then the papacy was passed on to Stephen (VI) VII. In January of 897, Lambert, his mother and Guy IV entered Rome and it is believed that what happened next was a form of revenge against Formosus on the part of Guy IV. Around that same time, Stephen (VI) VII ordered that the body of Pope Formosus be removed from his tomb and brought to the papal court for judgement. What happened next is now known as the Cadaver Synod and the story only gets stranger from here.