Using a national-scale citizen science framework we examined variation in mercury (Hg) concentrations of dragonfly larvae across 92 National Park Service units throughout the continental US and Alaska to facilitate their development and implementation as bioindicators of Hg availability in aquatic ecosystems at a range of spatial scales. Total Hg (THg) was an effective proxy for methylmercury (MeHg) concentrations (r² = 0.96) across sampling locations and dragonfly families. Across more than 400 unique sample sites, THg concentrations averaged 112 ng/g dry weight (dw) but ranged 135-fold, between 10.4 to 1,411 ng/g dw. Variation in THg concentrations was influenced by habitat type; concentrations were highest in riverine habitats, and lowest in ponds. Additionally, rivers, streams, lakes, and ponds with substantial peripheral wetlands had substantially higher THg concentrations than comparable habitats without adjacent wetlands. Total Hg concentrations differed among dragonfly families, but strong correlations between families facilitated robust conversion equations to normalize concentrations by family. We developed the Aeshnid-equivalent dragonfly Hg concentration to facilitate unbiased spatial and temporal comparisons of Hg concentrations in consistent units. Aeshnid-equivalent THg concentrations were positively correlated with THg concentrations of four fish guilds (piscivores, sunfish, trout, and forage fish), and two amphibian taxa (salamanders and frogs) indicating that dragonfly larvae are effective indicators of Hg availability in aquatic food webs. Using the relationships between dragonfly and fish Hg concentrations we developed a series of integrated impairment benchmarks that inform potential risk of Hg exposure to fish, wildlife, and human health. Across 402 sites in 92 National Park Service units, dragonfly THg concentrations indicated that 11% of sites did not exceed any impairment benchmarks, whereas 49%, 28%, 11%, and 1% of sites exceeded low, moderate, high, and severe categories for potential health impairment. These findings highlight the efficacy of carefully designed citizen science efforts to facilitate inquiry on issues of conservation significance that would be otherwise difficult to conduct because of their resource requirements. Additionally, it demonstrates the utility of applying dragonfly larvae as robust bioindicators across a broad landscape and diverse habitats in order to measure responses to changes in environmental Hg availability and estimate potential health risks across a range of relevant endpoints.