There is evidence that green space may reduce socioeconomic disparities in health (‘equigenesis’). However, it is still unclear whether such a finding is also valid for cognitive function in older adults. Therefore, using data from the Northern Ireland Cohort for the Longitudinal Study of Ageing, this study examines whether exposure to green spaces reduces the effect of socioeconomic disparities in cognitive function in later life. It uses the Normalised Difference Vegetation Index (NDVI) and accessibility of green space (distance to the nearest parks/woodland) as estimates of green space exposure. Cognitive function in later life was measured using the Mini-Mental State Examination (MMSE) scores. Life-course disparities were assessed in early life (self-reported childhood socioeconomic status and childhood unemployment of parents), and later life (education, Area-level Multiple Deprivation Measure and capital value of the housing), respectively. Multilevel linear regressions were used to model the associations among green space, life-course socioeconomic disparities and cognitive function. Results show that MMSE scores were positively associated with NDVI (Coef. = 0.51; SE = 0.13), but negatively associated with distance to the nearest parks (Coef. = −0.09, SE = 0.02). Also, participants with higher educational attainment, living in a property with a higher capital value or in less deprived neighbourhoods had higher MMSE scores. The association effects of green space exposure were stronger in older adults with lower socioeconomic status in early and later life. Our findings suggest that green space may reduce the life-course socioeconomic inequalities in cognitive function in later life.