How do some families manage to entrench themselves in the upper class for many generations while others do not? Bringing together economic sociology, political sociology, and stratification, I introduce a concept for the study of multigenerational persistence at the top of a stratified society: kinship interlocks. Kinship interlocks are portions of a kinship network that closely combine great wealth, status, and power. Just as board interlocks connect corporate elites through overlapping board memberships, kinship interlocks connect economic, social, and political elites through family ties. Using a mixed-methods analysis, I find that the intimate exchange of resources in kinship interlocks generates upper-class persistence via two primary mechanisms: it protects kin from economic, legal, and social risk, and it propels kin into higher strata. Processes of kin formation and intimate exchange are co-constitutive with systems of gender, sexuality, and race, such that the most durable portions of an upper class are especially heteronormative and racially dominant. The analysis is based on a unique dataset consisting of the full upper class and all economic, political, and social elites in the first 122 years of Dallas, Texas, along with all mutual family ties.