Although environmental racism is a fundamental cause of health inequities, measurements in the US are lacking. In this paper, we developed an area-based, scalable Environmental Racism Index (ERI) to measure the strength of the associations among neighborhood environmental quality, mortgage lending disinvestment, and racial/ethnic density across US geographies. The ERI was estimated for both Metropolitan Statistical Areas (MSAs) and states using 2015-2019 Census data, Home Mortgage Disclosure Act data, and various environmental databases. The associations between ERI and racial/ethnic mortality rates and rate ratios were examined at the MSA level. Findings illustrate substantial variations in environmental racism across the US, with MSAs in the Midwest and Northeast particularly severe. The ERI is positively associated with non-Hispanic (NH) White, NH Black, and Black mortality rates, as well as NH Black and Black to NH White mortality rate ratios at the MSA level. The ERI can be adapted for other geographic settings, time periods, and geographic scales to reveal patterns of environmental racism, measure its impacts on health inequities, and guide environmental and public health policy interventions to address these inequities.