Prester John was said to have been an Eastern Christian monarch who ruled over a fantastical Empire in the Far East, containing both marvels and treasure. Originating in the twelfth century, this legend captivated the mind of Europeans well into the Age of Exploration and was still a figure of interest in the eighteenth century. Accordingly, this study takes a necessary Longue durée approach to the reception history of Prester John. Key to this study is an examination of how the reception of Prester John is indicative of wider European conceptions of Eastern alterity. Figures of curiosity, such as Prester John, ‘existed’ in spaces of ignorance (Terra Incognita) that exhibited a different category of knowledge to that of the ‘known’ world, from the European perspective. As Europeans expanded their knowledge of the World, through travel and exploration, figures of curiosity were pushed further to the margins of geographical ignorance. This study examines how this epistemological conception of Prester John allowed him to be reimagined from being an Asian Crusader king to the Emperor of Ethiopia, and continue to be involved in western debates about the religious orthodoxy of Oriental Christianity from the Crusades into the Reformation. Consequently, this study calls for a new approach to myth and its push-pull relationship to scientific knowledge formation.