Modern coral reefs face substantial threats that endanger their capacity to function and provide resources for human populations. Chronic human pressure through exploitation and land-based pollution has led to reef degradation and lower productivity. More recently, this pressure is compounded with successive episodes of coral bleaching across the tropical oceans. However, response trajectories of reef resources to exploitation, habitat degradation, and local management may not be straightforward, as these ecosystems are incredibly complex. We employed a comprehensive and standardized survey of parrotfish assemblages within reef systems in Guam to examine trajectories of biomass and function across a decade of change. Parrotfishes represent one of the most commercially and ecologically important coastal resources throughout Micronesia. We found that parrotfish biomass declined by nearly 30% over the course of ten years, with concomitant decreases in grazing and bioerosion rates at a similar magnitude. There was evidence of fishery exploitation playing a role, but overall patterns were not always aligned with the hypothesis of fishery-induced decline. For example, two-thirds of marine-protected area sites declined in biomass and mean body length was stable for several highly targeted species. Further, most biomass decline stemmed from small species with higher resilience to fishery exploitation, whereby three of the most vulnerable fishery target species either maintained or increased total biomass. The ultimate drivers of parrotfish assemblage trajectories in Guam are likely a complex mixture of exploitation, habitat change from multiple stressors, and responses to management measures. However, the potential future decline in fishery production and ecological function is substantial and merits continuous monitoring and proactive management.