Aims: We investigated the mediating role of leisure activity engagement in the longitudinal relation between openness to experience and subsequent change in executive functioning over 6 years as measured through performance changes in the Trail Making Test (TMT). Methods: We analyzed longitudinal data from 897 older adults (mean = 74.33 years in the first wave) tested on TMT parts A and B in two waves 6 years apart. Participants reported information on leisure activity engagement and openness to experience. Results: Latent change score modeling revealed that 37.2% of the longitudinal relation between higher openness to experience in the first wave of data collection and a smaller subsequent increase in TMT completion time from the first to the second wave (i.e., a smaller decline in executive functioning) was mediated via a higher frequency of leisure activities in the first wave. Conclusion: Individuals with higher openness to experience show greater activity engagement in old age. By enhancing their cognitive reserve, this activity engagement may finally result in smaller subsequent decline in executive functioning.