Mercury (Hg) is a globally distributed environmental contaminant with a variety of deleterious effects in fish, wildlife, and humans. Breeding songbirds may be useful sentinels for Hg across diverse habitats because they can be effectively sampled, have well-defined and small territories, and can integrate pollutant exposure over time and space. We analyzed blood total Hg concentrations from 8,446 individuals of 102 species of songbirds, sampled on their breeding territories across 161 sites in eastern North America [geometric mean Hg concentration = 0.25 μg/g wet weight (ww), range <0.01–14.60 μg/g ww]. Our records span an important time period—the decade leading up to implementation of the USEPA Mercury and Air Toxics Standards, which will reduce Hg emissions from coal-fired power plants by over 90 %. Mixed-effects modeling indicated that habitat, foraging guild, and age were important predictors of blood Hg concentrations across species and sites. Blood Hg concentrations in adult invertebrate-eating songbirds were consistently higher in wetland habitats (freshwater or estuarine) than upland forests. Generally, adults exhibited higher blood Hg concentrations than juveniles within each habitat type. We used model results to examine species-specific differences in blood Hg concentrations during this time period, identifying potential Hg sentinels in each region and habitat type. Our results present the most comprehensive assessment of blood Hg concentrations in eastern songbirds to date, and thereby provide a valuable framework for designing and evaluating risk assessment schemes using sentinel songbird species in the time after implementation of the new atmospheric Hg standards.