Little is known of Formosus prior to his installation as cardinal (864) by Pope St. Nicholas I to oversee Porto, Italy as bishop. He was sent on a mission to France by Pope Adrian II in 869, and later by Pope John VIII in 875; for unknown reasons he earned the enmity of John in 876 and fled from Rome. His excommunication was lifted by Pope Marinus I, and he subsequently served once again at Porto. His influence and reputation grew under Marinus, St. Adrian III, and Stephan V; when Stephan died in 891, Formosus was elected pope.
Formosus became involved in the dispute for the crown of France between Udes of Paris and Charles the Simple. The political position in Italy directly affected the pope as head of the ecclesiastical estates, and consequently his independence as head of the Church. In addition, Emperor Guido of Spoleto, who was an oppressor of the Holy See and the papal territories, was too close to Rome to be ignored. Formosus secretly persuaded Arnulf, king of the east Franks, to advance to Rome and liberate Italy; in 894 Arnulf made his first expedition and subjugated that part of Italy north of the Po River. Guido died in December of the same year, leaving his son Lambert, whom Formosus had crowned emperor, in the care of his mother Agiltrude, the implacable opponent of the Carlovingians. In the autumn of 895 Arnulf undertook his second Italian campaign, and in February, 896, stood before the walls of Rome. Agiltrude had fortified herself in the city, but Arnulf succeeded in entering and was solemnly crowned by the pope. The new emperor thence marched against Spoleto to besiege Lambert and his mother, but was struck with paralysis on the way and was forced to abandon the campaign. On April 4, 896 Formosus died. He was succeeded by Boniface VI, who reigned a mere fifteen days.
Formosus made many political enemies as cardinal as well as pope, and the new pontiff, Stephan VI, ordered Formosus to be put on trial to face the revived charges brought under John VIII. His corpse was exhumed, dressed in the papal vestments, and propped in a chair; a deacon standing behind it answered for the deceased man. The outcome of the macabre "cadaver synod" was never in doubt; Formosus' election as pope was declared to be invalid, his vestments were torn off his body; the three fingers from which he pronounced his blessings were torn from his hand, and the corpse was dragged through the streets of Rome to be cast into the Tiber. The trial subsequently divided Rome, resulting in Stephan's imprisonment and eventual murder.
Formosus' letters and writings were collected into volume 129 of the Patrologia Latina.