A hallmark of the Anthropocene is the global expansion of pollution stemming from electric lighting. This evolutionarily novel phenomenon has left few spaces on Earth where natural light cycles remain unaltered. Assessing the exposure of species to light pollution is critical for developing conservation plans that address this expanding sensory pollutant. Here we used data from NASA's Visible Infrared Imaging Radiometer Suite to determine the contribution of full moonlight radiance to establish an ecologically-informed threshold of natural radiance. Using the geographic ranges of 351 mammals across the contiguous United States, we estimated the percentage of each species' range in which light pollution exceeded thresholds associated with direct emissions and skyglow, where it did not (“dark environment”), and the associated fragmentation of dark environments. Average mammal range contained 2.6% (95% CI: 2.3–3.2%) of area that consistently exceeded full moon radiance at point-source emissions, but with a large range among species (0–47.4%). Skyglow affected far greater percentages of ranges (X¯=24.3%; 95% CI: 22.1–26.8%). Nocturnal species had slightly greater exposure than diurnal species. Several families with the most impacted ranges included species of conservation concern (e.g., Molossidae; free-tailed bats). When assessing connectivity of dark environments, we found light pollution fragmented most mammal ranges and resulted in isolated dark refugia (e.g., 154% average increase in patches of dark environments). Identifying species with the greatest exposure to and dark environment fragmentation from light pollution is an important step for targeted conservation efforts of remaining dark refugia for light-sensitive species.