George MacDonald (1824-1905) is an important figure in the history of fantasy literature. His influence on the Inklings is well documented, and his works for children remain in print to this day. This thesis explores the extent to which fantasy is found in, and shapes, MacDonald's work. By highlighting this aspect of his thought the foundations of the genre of adult fantasy are exposed. The thesis uses Phantastes, MacDonald's first adult fantasy, and Lilith, his final work of adult fantasy, as the respective poles of his career to show the beginnings of a literary experiment and its conclusion. The study is undertaken chronologically, with particular attention paid to the substantial manuscript tradition of Lilith. This permits an exploration of the development of MacDonald's understanding of fantasy at the theoretical level and at the practical level in the act of redrafting. By dedicating entire chapters to these the core elements the then novel idea of adult fantasy is demonstrated as arising in no small part out of an intentional literary undertaking on the part of George MacDonald. By considering MacDonald as principally a literary figure, as opposed to a theological one, it sets out a theoretical framework relevant to the development of fantasy more generally. This is achieved by applying more recent theories on genre to MacDonald, and then considering him in tension with his sources and contemporaries.