Objective: Global increases in age means it is important to examine interventions and factors that can address age-related cognitive decline and its associated risk of dementia before cognitive impairment develops. This thesis aimed to investigate the relationship between mindfulness and subjective and objective cognitive function, as well as sleep quality and its role in the relationship between mindfulness and these outcomes, in healthy older adults. Method: Chapter 2 (Ch2) was a systematic review and meta-analysis of seven randomized-controlled trials (RCT) investigating mindfulness-based intervention’s (MBI) effect on objective cognitive function and sleep quality in healthy older adults. Chapter 3 (Ch3) was a secondary analysis of RCT data (SCD-Well trial) examining trait mindfulness, MBI, subjective cognitive function (SCF), and sleep quality as a mediator between MBI and SCF in healthy older adults with subjective cognitive decline (SCD). Results: Meta-analysis revealed no significant effect of MBI on objective cognitive function but demonstrated significant improvements in sleep quality in healthy older adults with non-clinical sleep difficulties. Ch3 regression analysis showed trait mindfulness was significantly associated with SCF. Repeated measures ANOVA found a significant effect of time on SCF, with no significant group difference. Mediation analysis showed sleep quality did not mediate the MBI-SCF relationship; baseline SCF significantly predicted follow-up sleep quality and follow-up SCF, and trait mindfulness significantly predicted follow-up SCF. Conclusion: MBI improves sleep problems in healthy older adults but is not more effective than active controls for objective cognitive outcomes. Although trait mindfulness is associated with better SCF, MBI is not superior to psychoeducation for improving SCF. Mediation results suggests the directionality of the sleep-SCF relationship may be more complex than hypothesised. Limitations include few eligible RCTs, incomplete RCT data access, and measurement heterogeneity (Ch2), potential ceiling effects in sleep (Ch3) and cognition (Ch2), and possible personality trait confounds. Findings have implications for early, longer duration MBIs to promote trait mindfulness and sleep quality in healthy older adults. Future research should investigate mindfulness and psychological, neural, or cognitive reserve outcomes rather than objective cognitive function in healthy older adults.