Between 1910-1930 and 1940-1970, millions of Black Americans left the U.S. South fleeing racial terror and seeking economic and political opportunity. However, northern destinations often responded with hostility – intensifying segregation, expanding policing, and disinvesting from public goods. The long-run consequences of these migration patterns for racial health inequality remain poorly understood.

We assemble a harmonized county-level mortality panel spanning 1959-2023 and estimate Black-White premature (<65) mortality inequities using a small-area Bayesian spatio-temporal model. We link these estimates to historical census data measuring the intensity of second Great Migration inflows (1940-70), defined as the net increase in Black population relative to total 1940 population. Northern commuting zones are divided into population-weighted quartiles of migration exposure and compared to the South. We find persistent monotonic relationship between migration intensity and long-run racial mortality inequity. In 1959, Black-White premature mortality rate ratios (RRs) were similar across northern quartiles (1.89-1.92), while the South exhibited higher inequity (RR=2.13). Over time, these trajectories diverged. By 2019, RRs were ordered by historical migration exposure: 1.26 in the lowest-inflow northern quartile, 1.45 in Q2, 1.71 in Q3, and 1.77 in the highest-inflow quartile, compared to 1.37 in the South. High-inflow northern areas experienced persistently elevated disparities and slower convergence.

These findings show that the geography of contemporary racial mortality inequality closely tracks historical migration exposure. The results are consistent with accounts suggesting that migration interacted with local institutional and spatial structures in ways that may have had durable implications for population health.