The northern United States long served as a land of opportunity for black Americans, but today the region’s racial gap in intergenerational mobility rivals that of the South. I show that racial composition changes during the peak of the Great Migration (1940-1970) reduced upward mobility in northern cities in the long run, with the largest effects on black men. I identify urban black population increases during the Migration at the commuting zone level using a shift-share instrument, interacting pre-1940 black southern migrant location choices with predicted out- migration from southern counties. The Migration’s negative effects on children’s adult outcomes appear driven by neighborhood factors, not changes in the characteristics of the average child. As early as the 1960s, the Migration led to greater white enrollment in private schools, increased spending on policing, and higher crime and incarceration rates. I estimate that the overall change in childhood environment induced by the Great Migration explains 43% of the upward mobility gap between black and white men in the region today.