Background <p>This study investigates the association between physical activity and cognitive function in adults aged 50 and older, with a focus on the mediating role of depressive symptoms.</p> Methods <p>Data were drawn from Waves 7 to 9 of the English Longitudinal Study of Ageing (ELSA), including 5,499 participants aged 50 years and older. Cognitive function was assessed through measures of episodic memory, numeracy, and orientation. Physical activity was quantified using a composite measure combining the frequency of vigorous, moderate, and light exercise with their respective average metabolic equivalents. Depressive symptoms were evaluated using the CES-D-8 scale. Generalized estimating equations (GEE) were employed to examine the association between physical activity and cognitive function, while the Bootstrap method was utilized to evaluate the mediating role of depressive symptoms.</p> Results <p>GEE analyses revealed that higher levels of physical activity were significantly associated with improved global cognitive function(<i>β</i> = 0.151, 95%<i>CI</i>: 0.118–0.183), episodic memory(<i>β</i> = 0.074, 95%<i>CI</i>: 0.058–0.090), numeracy(<i>β</i> = 0.033, 95%<i>CI</i>: 0.015–0.051), and orientation(<i>β</i> = 0.039, 95%<i>CI</i>: 0.022–0.057) (all <i>P</i> &lt; 0.001). Physical activity also demonstrated a significant negative association with depressive symptoms(<i>β</i>=-0.402, 95%<i>CI</i>: -0.452 to -0.352) (<i>P</i> &lt; 0.001), which, in turn, were linked to lower cognitive function. Notably, depressive symptoms mediated 8.46% of the association between physical activity and global cognitive function.</p> Conclusions <p>Greater engagement in physical activity is associated with better cognitive function, and this association is partially mediated by depressive symptoms. These findings underscore the importance of promoting physical activity to support cognitive health in older adults, with particular attention to its potential role in mitigating depressive symptoms.</p>