In bipolar disorder (BD), physical inactivity and sedentary behaviour are prevalent and have been linked to BD's cognitive symptoms, although the directionality of these links is not clear. This proof-of-concept study examined whether cognitive function during mid- and later-life was prospectively related to physical activity and sedentary time, and whether the association differed in presence or extent between those with BD and healthy controls. Relevant UK Biobank data were available for 646 BD participants and 18,041 psychiatrically healthy controls, aged 40-69 years at baseline. Cognition was assessed during a baseline assessment, and wrist-worn accelerometry data were collected at a follow-up assessment 2.8-6.6 years later. Regression analyses examined prospective relationships of global cognition, diagnostic group, and their interaction, with physical activity (total, light, and moderate to vigorous) and sedentary time. Baseline cognitive function was inversely associated with light physical activity (coeff. = -5.64, 95% CI: -6.30 to -4.98) and positively associated with sedentary time (coeff. = 5.17, 95% CI: 4.48-5.86) and moderate-to-vigorous physical activity (coeff. = 0.48, 95% CI: 0.28-0.68) at follow-up. Observed effect sizes were small but significant. In general, associations were not moderated by age or diagnostic group. The current study provides preliminary evidence that cognitive function may influence subsequent physical activity and sedentary time similarly in those with BD and healthy controls; however, further research is needed to confirm and further explore this findings.