William Morris has had many legacies: two of the most significant are in the fields of modern design and modern fantasy literature. In each, he had influential champions. The scholar Nikolaus Pevsner acclaimed Morris as a “pioneer of modern design” in 1936, and the fantasy author and critic Lin Carter lauded Morris as the progenitor of the “imaginary world” tradition of modern fantasy beginning in the 1960s. This chapter assesses their arguments through examinations of how modern design in interwar England came to be defined as an outgrowth of the Arts and Crafts movement, and how modern fantasy became associated in postwar North America with the creation of realistic yet autonomous “imaginary worlds” such as those found in Morris’s late prose romances. Morris’s fusion of medievalism and modernism assumed novel afterlives in each of these domains, as did his passion for world-building in actuality and in fiction.