Large-scale assessments are valuable in identifying primary factors controlling total mercury (THg) and monomethyl mercury (MeHg) concentrations, and distribution in aquatic ecosystems. Bed sediment THg and MeHg concentrations were compiled for > 16,000 samples collected from aquatic habitats throughout the West between 1965 and 2013. The influence of aquatic feature type (canals, estuaries, lakes, and streams), and environmental setting (agriculture, forest, open-water, range, wetland, and urban) on THg and MeHg concentrations was examined. THg concentrations were highest in lake (29.3 ± 6.5 μg kg^− 1) and canal (28.6 ± 6.9 μg kg^− 1) sites, and lowest in stream (20.7 ± 4.6 μg kg^− 1) and estuarine (23.6 ± 5.6 μg kg^− 1) sites, which was partially a result of differences in grain size related to hydrologic gradients. By environmental setting, open-water (36.8 ± 2.2 μg kg^− 1) and forested (32.0 ± 2.7 μg kg^− 1) sites generally had the highest THg concentrations, followed by wetland sites (28.9 ± 1.7 μg kg^− 1), rangeland (25.5 ± 1.5 μg kg^− 1), agriculture (23.4 ± 2.0 μg kg^− 1), and urban (22.7 ± 2.1 μg kg^− 1) sites. MeHg concentrations also were highest in lakes (0.55 ± 0.05 μg kg^− 1) and canals (0.54 ± 0.11 μg kg^− 1), but, in contrast to THg, MeHg concentrations were lowest in open-water sites (0.22 ± 0.03 μg kg^− 1). The median percent MeHg (relative to THg) for the western region was 0.7%, indicating an overall low methylation efficiency; however, a significant subset of data (n > 100) had percentages that represent elevated methylation efficiency (> 6%). MeHg concentrations were weakly correlated with THg (r² = 0.25) across western North America. Overall, these results highlight the large spatial variability in sediment THg and MeHg concentrations throughout western North America and underscore the important roles that landscape and land-use characteristics have on the MeHg cycle.