Abstract Objectives Neighborhood built environments (BEs) are increasingly recognized as being associated with late-life depression. However, their pathways are still understudied. This study investigates the mediating effects of physical and social activities (PA and SA) and functional ability (FA) in the relationships between BEs and late-life depression. Methods We conducted a cross-sectional analysis with data from 2,081 community-dwellers aged 65 years and older in Hong Kong in 2014. Two road-network-based service area buffers (200- and 500-m buffers) adjusted by terrain and slope from participants’ residences were created to define the scope of neighborhoods. BEs comprised population density in District Council Constituency Areas, urban greenness, land-use diversity, and neighborhood facilities within 200- and 500-m buffers. Multilevel path analysis models were used. Results More urban greenness within both buffers and more commercial facilities within a 500-m buffer were directly associated with fewer depressive symptoms. SA mediated the relationship between the number of community facilities and depressive symptoms within a 200-m buffer. Neighborhood urban greenness and the number of commercial facilities had indirect associations on depressive symptoms within a 500-m buffer, which were mediated by FA. Discussion Our findings have implications for the ecological model of aging. The mediating effects of SA and FA underscore the importance of promoting active social lifestyles and maintaining FA for older adults’ mental health in high-density cities. Policy implications on how to build age-friendly communities are discussed.