Physical performance in older age predicts independence, and social factors influence physical function, yet the biological mechanisms are not well understood. Mitochondrial function—a new “hallmark of aging”—may explain how social experiences impact health, as its dysfunction is linked to multiple age-related diseases and declining physical performance. However, no field studies have examined links between mitochondria, physical function, and social determinants in aging populations. This project leverages a new method to measure mitochondrial bioenergetics from frozen blood samples, enabling analysis with existing large cohort data from the Health and Retirement Study. Our aims are to assess the relationships between socioeconomic status, mitochondrial capacity, and physical performance, and to test if mitochondrial health connects socioeconomic status to functional outcomes. This research may reveal a biological pathway linking social and health factors in aging.