Peatlands are sources of the bioaccumulating neurotoxin methylmercury (MeHg) and linked to adverse health outcomes, yet the impact of climate change and reductions in atmospheric pollutants on mercury (Hg) export from peatlands are highly uncertain. Here, we present the response in annual flow-weighted concentrations (FWC) and yields of total-Hg (THg) and MeHg to cleaner air and climate change using an unprecedented hydroclimatic (55-years; streamflow, air temperature, precipitation, regional and peatland water tables), depositional chemistry (21-years; Hg and major ions concentration and total mass), and streamwater chemistry (~17-years; THg, MeHg, major ions, total organic carbon, and pH) datasets from a reference peatland catchment in the north central USA. Over the hydroclimatic record, annual mean air temperature increased by ~1.8 ℃, decreasing baseflow and, subsequently, the efficiency that precipitation was converted to streamwater runoff (runoff ratio). Concurrently, precipitation-based deposition of sulfate and Hg declined, where wet Hg deposition rates declined to near pre-industrial levels. Annual MeHg FWC was positively correlated mean annual air temperatures (p=0.03, r=0.51), annual runoff ratio (p<0.0001, r=0.76), and wet Hg deposition concentration (p<0.0001, r=0.79). Over the study period, decreasing wet Hg deposition concentration and annual runoff ratios counterbalanced increased peatland MeHg production due to higher air temperatures, leading to an overall decline in streamwater MeHg FWC. Climate change and cleaner air were responsible for 0.51 and 0.32 of the variability in MeHg FWC, respectively. Streamwater MeHg export may continue to decrease only if declines in runoff ratio and wet Hg deposition concentration persistently outpace increased air temperature.