This study attempts to reconstruct Proto-Boro-Garo (PBG), the ancient language from which the modern Boro-Garo (BG) family evolved. BG is a largely under-documented sub-branch of Tibeto-Burman that is spoken primarily in the Brahmaputra valley of northeastern India. While other comparative studies have focused on PBG phonology, this study concentrates on grammatical elements and syntactic structures. An initial reconstruction is attained by examining data from the limited number of descriptive grammars available on BG languages and using the comparative method to determine the oldest forms of grammatical elements. Where elements correspond across languages, they can be reconstructed for the common ancestor. When they do not, we have evidence for independent innovation. This is accounted for, when possible, by language-internal reconstruction.