Previous studies have reported multifaceted, controversial social outcomes of densely built urban settings. Social sustainability of urban environments have rarely been studied in a context-sensitive manner, identifying the specific ways urban structural characteristics contribute to the behavioural, experiential and well-being outcomes. In this study, an online public participation geographic information system (PPGIS) methodology allowed the place-based study of urban and suburban contexts in the metropolitan region of Helsinki, Finland. Respondents (N = 3119) located their meaningful places and reported the experiential and well-being outcomes. GIS-based measures of urban structures were calculated within a 500m buffer around their homes. Structural equation modeling was used to assess the contextual variation and the mediational role accessibility and perceived environmental quality play in linking urban structural characteristics with well-being outcomes. Our findings indicated that although increasing urban density was associated with shorter distances to everyday services in both urban and suburban settings, the experiential and well-being outcomes varied. In the urban context, easy access to services contributed to higher perceived environmental quality and positive well-being outcomes, whereas in the suburban setting, the closeness of services decreased the experiential and well-being outcomes. Perceived environmental quality was strongly associated with well-being in both contexts. We concluded that densely built urban neighborhoods can also support social sustainability, but the processes vary between suburban and urban settings. A challenge remains for urban planners on how to improve accessibility and related positive experiential outcomes in suburban contexts.