This paper shows that racial composition shocks during the Great Migration (1940-1970) reduced the gains from growing up in the northern United States for Black families and can explain 27 percent of the region's racial upward mobility gap today. I identify northern Black share increases by interacting pre -1940 Black migrants' location choices with predicted southern county out-migration. Locational changes, not negative selection of families, explain lower upward mobility, with persistent segregation and increased crime and policing as plausible mechanisms. The case of the Great Migration provides a more nuanced view of moving to opportunity when destination reactions are taken into account. (JEL H75, H76, J15, J62, K42, N32, R23)