This dissertation challenges dominant narratives of Buddhism’s abrupt decline in Eastern India by offering a regionally grounded and materially informed reassessment. Focusing on a subregion of Magadha—Nalanda-Bihar Sharif (NBS) in southern Bihar—it employs an interdisciplinary approach that draws on archaeology, art history, epigraphy, medieval Indian history, oral traditions, and colonial-era records. The study begins by critically reevaluating the historiography of Buddhist decline, revealing how colonial-era assumptions and the uncritical use of sources such as Persian chronicles, Tibetan accounts, and early archaeological interpretations, particularly from Nalanda, have reinforced a reductive narrative of sudden collapse and persecution-driven displacement. In response, this dissertation incorporates new evidence gathered through extensive fieldwork, such as material remains, regional topography, and oral traditions. Focusing on the period from the thirteenth to the sixteenth centuries, with contextual forays beyond where necessary, it traces shifting patterns in the built environment, landholdings, and socio-religious dynamics. These transformations challenge the conventional association of Buddhist decline with a singular episode of destruction. Instead, the evidence points to a more complex picture of gradual change, shaped by new landholding patterns, especially under the Tughluq administration in the fourteenth century, and the rise of new social and religious groups aligned with this changing political configuration. By tracing both the transformations and challenges experienced by Buddhist sites and their custodians during this extended period, the dissertation reframes Buddhist institutions in Eastern India around the concept of monastic landlordism, a system that gradually eroded in the face of shifting landholding claims, changing patterns of patronage, and the emergence of new institutional competitors. It proposes an alternative model for understanding the trajectory of medieval Buddhist history, one rooted in regionally specific developments while remaining attentive to broader transregional connections, shifting over time from affiliations with Bengal to deeper integration into networks tied to Delhi and Western India in the late medieval period. Ultimately, this dissertation advances a new framework for interpreting the longue durée histories of religious transformations in South Asia. By centering a microregional approach, it advocates for a more layered, non-linear, and heterogeneous understanding of religious change—one attentive to the lived realities of traditions, their custodians, and the spaces they shaped and inhabited