Gregory III Mammê, pro-union patriarch of Constantinople between 1445 and 1459, has been neglected by historians. Because he fled from Constantinople after only five years in office and spent the rest of his life in exile in Rome, he might seem irrelevant to the development of events. This article argues that his career is, on the contrary, very instructive. His actions will be placed in the context of the reaction of the late Byzantine elite to the predicament in which they found themselves in the last years before the fall of Constantinople. It will examine the patriarch’s participation in diplomatic initiatives to attract western help against the Ottomans, his efforts to fill certain roles that the emperor was unable to play and his ambivalence about relations with the Catholic Church. It will argue that Gregory’s actions typify those of his class at the time: a combination of heroic self-sacrifice, cynical diplomacy and calculated self-interest.