Gregory X (baptismal name Teobaldo Visconti) was born around 1210 to a noble Piacentine family — kinsman of the famous Milanese Visconti dynasty. Educated at the cathedral schools of Piacenza and Lyon, he became archdeacon of Liège.
Teobaldo accompanied Cardinal Ottobuono Fieschi (the future Pope Adrian V) as papal legate to England (1265–1268) during the Barons' War of King Henry III. He subsequently undertook the pilgrimage to the Holy Land in 1271 — arriving at Acre in the company of Prince Edward of England (the future King Edward I).
During the Ninth Crusade at Acre, the news arrived that the papal see had been vacant for nearly three years — since the death of Pope Clement IV on November 29, 1268. The cardinals at Viterbo had been deadlocked in the contested election for over thirty months (producing the famous forced-conclave narrative: the city magistrates of Viterbo had locked the cardinals in the papal palace, torn off the roof, and reduced them to bread-and-water — the literal origin of the term 'conclave', Latin 'cum clave' or 'with a key').
On September 1, 1271, the deadlocked cardinals finally elected Teobaldo Visconti (who was not himself a cardinal, not even in priestly orders at the time — the classic 13th-century compromise-candidate election).
Teobaldo accepted the election and returned to Italy — taking the papal name Gregory X. His pontificate (1271–1276) was devoted to the planning of a new Crusade to the Holy Land, the Second Council of Lyon (May–July 1274), and the institution of the papal conclave procedure.
The Second Council of Lyon (May–July 1274) — convened by Gregory X — undertook the reunion between the Latin-Catholic Western Church and the Greek-Byzantine Eastern Church. Byzantine Emperor Michael VIII Palaeologus sent his delegation to the Council, and on July 6, 1274 the union between the two Churches was formally proclaimed — the first attempted Catholic-Orthodox reunion at the ecumenical-council level (not sustained past the death of Michael VIII in 1282, but institutionally significant as the first serious Catholic-Orthodox ecumenical reunion attempt).
Gregory X also promulgated at the Council the bull 'Ubi Periculum' (July 7, 1274) — the famous institution of the papal conclave: after the three-year deadlock at Viterbo, Gregory X decreed that at the death of a pope, the cardinals must gather in the palace within ten days, be locked into a single chamber (with only strictly necessary attendants), receive only one dish of food per day (reduced to bread and water after eight days), without personal communication with the outside world, until the election of a new pope. The Ubi Periculum conclave procedure (with modifications) remains the basic institutional structure of the papal conclave to the present day.
Gregory X died at Arezzo on January 10, 1276, on the return journey from the Second Council of Lyon to Rome.