Peter Thomas (French Pierre Thomas) was born around 1305 to a French Périgourdine peasant family at Salle-de-Sarlat in Périgord. Educated as a young man at the Carmelite houses and at the Universities of Paris and Bologna, he entered the Carmelite Order and received the Doctor of Theology.
Peter Thomas served in multiple Carmelite provincial offices and was called by Pope Innocent VI to Avignon in 1354 as Procurator General of the Carmelite Order.
From 1355 to 1366 — under Popes Innocent VI, Urban V — Peter Thomas served as the principal papal-diplomatic legate to the Byzantine and eastern-Catholic and Slavic Christian frontiers. His diplomatic missions included: Serbia (1355), Byzantine Constantinople (1357), Hungary (1359), Byzantine Constantinople again (1359–1360), and Alexandria (1364–1365).
In 1359, Peter Thomas was appointed Bishop of Patti and Lipari; subsequently Archbishop of Crete (1363), and finally Latin Patriarch of Constantinople in 1364 (the Latin-Catholic Patriarchate of Constantinople established after the 1204 Fourth Crusade — residing in the Crusader-Aegean territories after the 1261 Byzantine reconquest of Constantinople).
As Latin Patriarch of Constantinople, Peter Thomas undertook the mission of the reunion between the Latin-Catholic Western Church and the Greek-Byzantine Eastern Church — in institutional continuity with the earlier Second Council of Lyon (1274) reunion attempt.
In 1364–1365, Peter Thomas undertook the Alexandrian Crusade with King Peter I of Cyprus — the last significant medieval Catholic Crusader expedition to the eastern Mediterranean. The Crusader army captured Alexandria on October 9, 1365 but was forced to evacuate the city on October 16, 1365.
Peter Thomas received wounds during the unsuccessful siege of Alexandria and returned to Famagusta in Cyprus, where he died of his wounds on January 6, 1366.

