Objective: Evidence points to an association between social and leisure activity (SLA) engagement and cognitive outcomes, but the mechanisms underlying this link remain unknown. We aimed to investigate three potential mechanisms: Vascular function, Perceived Stress, and Cognitive Reserve. Methods: With data from 8163 adults aged over 50 in the Irish Longitudinal Study of Ageing, we used a structural equation model to evaluate Vascular Function and Perceived Stress as potential mediators, and Cognitive Reserve as a potential antecedent in the relationship between SLA at baseline (2009), and cognitive outcomes collected at a two-year follow-up point (2011). Results: Cognitive Reserve was strongly associated both with cognitive outcomes (β = 0.306; p < 0.001) and with SLA (β = 0.694; p < 0.001). Perceived stress (β = 0.018) acted as a significant mediator in the relationships between SLA and cognitive outcomes (p < 0.001), although Vascular Function did not (β = 0.000). Conclusion: These results indicate that SLA may protect cognitive function partly because of its association with cognitive reserve, and partly through its impact on perceived stress. Results have policy implications for those interested in facilitating SLA to protect cognitive outcomes among older adults.