Recent advances in isotope biogeochemistry are reshaping interpretations of the social roles animals played within ancestral Mesoamerican communities. Dogs (Canis familiaris) appear prominently in archaeological, ethnohistoric, and iconographic records, from Formative figurines of humans holding puppies to later accounts of dogs guiding the dead into the underworld. Yet the daily experiences that shaped these roles remain poorly understood. Isotopic analyses, which reveal what dogs ate and where they originated, offer a direct view of their lives and their relationships with the people around them. This study presents new isotopic data from dogs in the upper Belize River Valley, dating from the Preclassic through Terminal Classic periods (approximately 1000 BC to AD 900/1000). Preclassic dogs exhibit two distinct dietary patterns: some consumed wild-like diets, which suggests free-ranging or feral behavior, while others were closely provisioned by people. Strontium values from Preclassic teeth also identify several non-local individuals, indicating long-distance movement potentially undertaken alongside human companions. By the Classic period, dogs appear more frequently in burials, caches, and peri-abandonment deposits associated with pilgrimage. The prominence of non-local, maize-fed dogs in these contexts suggests intentional provisioning and circulation for ceremonial purposes. As one of the largest isotopic studies of Mesoamerican dogs, this research demonstrates how canine diets, mobility, and treatment illuminate broader social and political transformations.