This dissertation adopts an interdisciplinary approach informed by medieval Indian history, historical linguistics, and sociolinguistics to reconsider debates over Hindi-Urdu’s origins. Two of its animating questions are: 1) How did the local speech of Delhi, a modestly sized town of regional importance in the 12th century, survive all of the dramatic transformations of the 13th century to prevail as the general language of the Delhi of the early 14th century, a global metropolis whose population must have originated in large part through immigration from linguistic regions outside of the subcontinent? 2) What accounts for the starkly different sociolinguistic outcomes in two mass migration processes separated by roughly a century—no immigrant communities to north India around the turn of the 13th century ultimately retained their original languages, whereas 14th-century immigrants from north to peninsular India established Dakani as a language that would be maintained to the present day? The first half of the dissertation adopts a new methodology that synthesizes dialectological and traditional historical linguistic approaches to engage with existing origin accounts and establish the premises of the first question. Hindi-Urdu descends from the pre-Ghurid variety of the city of Delhi; north India was characterized not by linguistic uniformity or fluidity in this period but rather by vernacular pluralism; Hindi-Urdu did not originate through a process of mixing of varieties brought into novel contact in Delhi; seemingly anomalous early features can be accounted for without recourse to these explanations. The second half of the dissertation considers both questions through the framework of language maintenance and shift. It presents a new account of Hindi-Urdu’s origins that examines the variety’s history as it intersects with the multiple dramatic developments of the late 12th to 14th centuries—the Ghurid conquest of north India, the establishment of the Delhi Sultanate, the establishment of Islam in India’s midland north, and the transformation of Delhi itself. More broadly, the dissertation argues for a new approach to understanding Hindi-Urdu’s history as that of a regional variety’s transformation into an interregional lingua franca.