Research on social mobility has paid considerable attention to links with inequality. The vast majority of this work, however, has focused on ‘vertical’ inequality between individuals or households, rather than ‘horizontal’ inequality between groups in society. This chapter speaks to the latter, with focus on the persistence over time of horizontal inequalities between ethnic and culturally defined groups. Not only is such persistence due to low intergenerational mobility overall, we argue, it also is due to lower mobility for disadvantaged as compared with advantaged groups in many societies. Group-based discrimination, among other factors, contributes. We posit further that the relationship between low mobility and persistent horizontal inequality is a comparatively larger problem for Global South as compared to Global North countries, because they are, on average, contexts with lower intergenerational mobility, higher horizontal inequality, and weaker state capability and the rule of law.