Racial and ethnic minorities are substantially less likely to receive timely dementia diagnoses, yet the factors underlying these gaps remain poorly quantified. Using nationally representative Health and Retirement Study data linked to Medicare claims and National Neighborhood Data Archive, we examine racial and ethnic disparities in timely dementia diagnosis among U.S. older adults and decompose these gaps using causal mediation analysis. Controlling for demographics and health conditions, non-Hispanic Black and Hispanic individuals are less likely than non-Hispanic White individuals to receive a timely diagnosis. Educational attainment is the dominant mediator, explaining 48% of the Black–White disparity and 62% of the Hispanic–White disparity, followed by neighborhood affluence (27% and 18%, respectively) and the density of non-physician health practitioner offices (16% and 15%) and physician offices (10% and 12%). Dementia specialist evaluation accounts for a further 7% and 6%, respectively. These findings identify education and neighborhood healthcare infrastructure as the primary structural determinants of racial and ethnic gaps in dementia detection, pointing to targeted policy interventions to advance diagnostic equity.