This poster examines how social isolation and loneliness independently and jointly influence functional decline among older adults, with a focus on mobility, self‑care, and household activities. Using three waves of NHATS data (2021–2023), the study applies lagged generalized estimating equations stratified by cognitive impairment status. Results show that both social isolation and loneliness predict functional limitations, and their antagonistic interaction suggests non‑additive joint effects. Findings highlight the importance of integrating social health screening into geriatric assessments and developing interventions that combine mobility support, caregiver involvement, and meaningful social engagement to maintain independence in later life.