This thesis critically examines the earliest creative work of J.R.R. Tolkien, from which the first version of his mythology would emerge, as one coherent whole, rather than a series of individual creative acts. It argues that all aspects of Tolkien's creativity worked in a dialectic way to bring to life an invented secondary world the complexity of which fantasy literature had not seen before. In examining Tolkien's early creative process this study also offers an alternative profile and assessment of J.R.R. Tolkien, in contradistinction to the popular image of him as the elderly Oxford don, by critically reading him as a young man, student, budding philologist, soldier and World War One survivor. The scope of this thesis is a holistic examination of Tolkien's earliest creative output comprising poetry, prose, language invention and visual works and includes analysis of several of Tolkien’s early creative works which remain either unpublished or under-analysed. The study uses several contextual frameworks to offer an in-depth analysis of Tolkien’s early imaginative language invention, a neglected area in Tolkien studies, in spite of being at the core of Tolkien’s creative process. This thesis, therefore, is critically responding to a gap in Tolkien and fantasy literature scholarship, and offers new insights on the earliest writing phases of one of the most influential fantasy authors of the 20th century. The introductory chapter presents an overview of Tolkien criticism and defines the scope and range of the thesis. Chapter two examines how myth-making and language invention came together in Tolkien’s earliest works and argues that these two key elements become inextricably intertwined in the first full expression of Tolkien’s early mythology, The Book of Lost Tales. Chapter three explores the underlying religious underpinnings of Tolkien's mythology and his early attempt to employ overt Roman Catholic words and ideas into his emerging secondary world. This chapter goes on to demonstrate how Tolkien combined Roman Catholic ideas with elements of both pagan mythology and Victorian spiritualism into the fabric of his secondary world. Chapter four focuses on the role of visual expression in Tolkien's early mythology by reading two major groups of documents from this period: published drawings and paintings in which Tolkien expressed his early mythic ideas; and a group of visually oriented ‘para-textual’ elements, such as maps, charts and samples of Tolkien’s invented writing systems. These visual representations are explored as ‘trans-medial’ components which, along with layered narratives and language invention, make up the fabric of Tolkien’s invented secondary world. The last chapter of this thesis explores several ways Tolkien experimented with in order to link his growing body of mythology to the primary world. It examines Tolkien’s first ‘framework’ of transmission which relied on dreams, and dream vision, to attempt this link. The second half of this chapter explores how Tolkien developed a parallel narrative transmission ‘framework’ through the re-imagining and re-purposing of Germanic myth and legend. The thesis also includes a series of appendices: a chronology outlining Tolkien’s creative works from this time; a list of the books he borrowed from the Exeter College Library as an undergraduate; a detailed list of examples of Tolkien's early language invention from the time; and a transcript of a report on the literary talk Tolkien gave at Exeter College on the Anglo-Catholic poet Francis Thompson.