Abstract Objectives In this study, we examined the extent to which older adult social activity participation and perceptions of neighborhoods correspond with risks of cognitive impairment with no dementia (CIND) and dementia. Methods We predicted the risk of both CIND and dementia in a series of Cox proportional hazards analyses among older adults across a 10-year period. Utilizing data from the Health and Retirement Study (HRS, N = 15,020), we examined whether social activity participation corresponded with reduced risk of CIND and dementia, as well as whether perceptions of neighborhood conditions, social cohesion, and neighborhood disorder moderated the effects of social activity participation. Results The results showed that the hazards of both CIND and dementia were reduced with successive increases in social activity participation. However, those risks, particularly for dementia risk, were further reduced when older adults perceived their neighborhoods to be more cohesive (hazard ratio [HR] = 0.75). On the other hand, older adults who perceived their neighborhoods to have high disorder were observed to have an amplified risk of dementia (HR = 1.36). Discussion The findings suggest that social activity participation confers important protections against cognitive decline and dementia risk; however, such protections may also be conditional on perceptions of neighborhood characteristics. Thus, how older adults interact with their social environments, understood as a meso-scale interaction that links micro and macro systems, can characterize the impacts and opportunities of individual social and health behavior.