Objectives This study investigates heterogeneity in craniometrically‐derived estimates of admixture in order to reveal population substructure in a sample of Black, White, Hispanic, and Native American individuals from the FDB. It reports evidence of spatial trends in population‐specific patterns of admixture and contextualizes its results in terms of demographic diversity in the United States. Materials and Methods The FDB was sampled to capture the population variation within forensic casework, skeletal collections, and the U.S. population‐at‐large. Individuals were selected for the availability of population identifier, sex, and geographic information. Variation in inferred admixture proportions was evaluated, per population and by sex, for evidence of geographic substructure. Comparative data was sourced from the U.S. Census. Results This analysis identifies significant associations between the estimated Black, Native American and White component memberships and place of birth and recovery. The sampled populations differ significantly in admixture proportions, in a systematic way. Admixture patterns vary in accordance with the densities and relative proportions of the U.S. census populations. Discussion There is considerable variation in admixture estimates, not just between, but notably within, all four of the populations. This substructure can be explained by differences in geography, including regions, divisions, and states. This article's findings agree with census trends and speak broadly to admixture dynamics and ancestral diversity among contemporary Americans. They are also specifically relevant to those cases in the FDB. The presence of subpopulations has implications for cranial research, forensic identification, and studies of biological variation in the United States.