There are large differences in premature mortality in the United States by race/ethnicity, education, and rurality. It is also known that exposure to particulate air pollution (PM2.5) varies strongly across these population groups. Thus far, however, studies have not estimated to what degree the mortality disparities between these population groups can be attributed to differential PM2.5 exposure and related impacts, and how this has varied over time and across geography. Using tract-level data spanning 1990-2016, we examine variation and time trends in mortality attributable to PM2.5 exposure by race/ethnicity, education, and rurality in the US. We show that differences regarding mortality attributable to PM2.5 were consistently most pronounced between race-ethnicities as compared to education or rurality, with the Black population having by far the highest proportion of deaths that were attributable to PM2.5 in all years. Over half of the difference in age-adjusted all-cause mortality between the non-Hispanic White population and Black population was attributable to differential PM2.5 exposure in the years 2000 to 2015. Our study provides the most comprehensive evidence to date of the role of PM2.5 exposure in explaining mortality inequalities between sociodemographic groups in the United States over the past decades.