There have been recent advances in the phonological reconstruction of the South-Central (“Kuki}-Chin”) branch of Trans-Himalayan (Tibeto-Burman), in particular by VanBik (2009). However, the Northwestern (“Old} Kuki”) subgroup, generally considered to be conservative, is not represented in this work as reliable data have not been available. The present study provides a comprehensive documentation of the historical phonology of one Northwestern language, Monsang. The unexpected finding is that Monsang cannot be considered conservative in its phonological development. A large number of sound changes have occurred across all phonological domains. The majority of sound changes are mergers, and with small exceptions, no unusual sound changes are found. As a result, the diachronic development of Monsang can be considered a case of reduction in phonological complexity.