This article explores the relation between transatlantic immigration and wealth accumulation in late colonial Mexico City, the chief destination for transatlantic emigrants from Spain. In contrast to the prevalent focus in the literature on networks and transatlantic ties, I argue that economic mobility and thus entry into the upper classes of colonial society depended on the social background of immigrants in their home regions in the peninsula. Human capital rather than social capital was the key determinant explaining why certain regional groups, notably the Basques, succeeded economically as merchants and miners, challenging traditional notions of networks and interconnectedness in Atlantic history and beyond. The findings are grounded in quantitative datasets of Spanish immigrants, overcoming both the biases of anecdotal sources and the methodological intricacy of disentangling the causal relationship between wealth accumulation and social ties.