This paper examines the relative contributions of siting decisions and post-siting demographic shifts to current disparities in exposure to polluting fossil-fuel plants in the United States. Our analysis leverages newly digitized data on power plant siting and operations from 1900-2020, combined with spatially resolved demographics and population data from the U.S Census from 1870-2020. We find little evidence that fossil-fuel plants were disproportionately sited in counties with higher Black population shares on average. However, event study estimates indicate that Black population share grows in the decades after the first fossil-fuel plant is built in a county, with average increases in Black population share of 4 percentage points in the 50-70 years after first siting. These long-run demographic shifts are driven by counties that first hosted a fossil-fuel plant between 1900-1949. We close by exploring how these long-run demographic shifts were shaped by the Great Migration, differential sorting in response to pollution, and other factors. Our findings highlight that the equity implications of siting long-lived infrastructure can differ dramatically depending on the time span considered.