The Common Loon (Gavia immer), a top trophic-level piscivorous predator, was used as an indicator species to assess mercury exposure and risk in aquatic ecosystems in the Adirondack Park of New York State. Mercury levels in Common Loons were related to long-term reproductive success to evaluate the effects of mercury contamination on the breeding population in the Park and enable the development of a mercury hazard profile. Common Loons were sampled and monitored on selected study lakes from 1998–2007. Lake acidity correlated with Common Loon mercury levels, with more acidic lakes exhibiting higher mercury concentrations in Common Loons. Based on mercury body burden estimated by blood mercury exposure, 21% of males and 8% of females were at high risk for behavioral and reproductive impacts, while feather mercury exposure estimated that 37% of males and 7% of females were at high risk. Female and male Common Loons in the highest exposure category showed a 32% and 56% reduction, respectively, in the number of chicks fledged per year, compared to individuals in the lowest exposure category. Thirteen percent of the Adirondack Common Loon eggs sampled were at high risk for mercury exposure. Population model results indicated that the portion of the Adirondack Common Loon population with high mercury levels has a reduced growth rate (λ = 1.0005), compared to Common Loons with low body burdens of mercury (λ = 1.026). The results of this project will assist in the continued refinement of State and Federal policies and regulations that effectively address the ecological impacts mercury and other environmental contaminants pose to freshwater ecosystems. Received 21 January 2013, accepted 26 May 2013.