Background Atrial fibrillation (AF) is associated with declines in physical function. While physical activity is linked to better physical function in the general population, its long-term impact in people with AF remains unclear. Investigating this relationship could provide insights and inform interventions for this population. Methods 624 participants with AF from the Atherosclerosis Risk in Communities (ARIC) cohort assessed in 2011-2013 were studied. Physical activity was assessed using the modified Baecke Physical Activity Questionnaire. Physical function was measured using the Short Physical Performance Battery (SPPB), grip strength, and 4-meter walk time up to 3 times over an 8-year period, with 4-meter walk speed as a secondary outcome evaluated in supplemental analyses. Confounder-adjusted linear mixed models were used to assess associations between physical activity and change in physical function trajectories over time. Results Participants had a mean age of 78.5 {+/-} 5.4 years, with 52.6% males and 13.8% Black. Median follow-up was 6.6 years. At baseline, greater sport-related leisure time, non-sport leisure time, and total moderate-to-vigorous physical activity (MVPA) were cross-sectionally associated with better physical function. However, physical activity measures were not significantly associated with temporal trajectories in physical function over time. Conclusions In participants with AF, greater habitual physical activity was significantly associated with better baseline physical function but not with future trajectories. Randomized trials are needed to examine whether interventions that improve habitual physical activity or MVPA can improve physical functioning in individuals with AF.