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.