Against the backdrop of population aging, community public spaces are important neighborhood settings for older adults’ daily activities, social ties, and mental well-being. However, few studies have integrated multidimensional perceived environmental attributes, parallel physical-activity and social-participation pathways, and recent positive mental well-being within a single framework. A cross-sectional survey involved 842 adults aged 60 years or older from 15 communities in Shenyang, China. Perceived environmental attributes comprised Functional Supportiveness, Perceived Restorativeness, and Social Interaction Support, with Neighborhood Outdoor Physical Activity and Community-Based Social Participation specified as parallel mediators and mental well-being assessed using the WHO-5 Well-Being Index. Analyses included confirmatory factor analysis, structural equation modeling, bootstrap tests, and multigroup analysis. All three environmental attributes were significantly and positively associated with both community activities and mental well-being, with significant indirect effects through both activity pathways. Among the three environmental attributes, Perceived Restorativeness showed the strongest associations with Neighborhood Outdoor Physical Activity and mental well-being, whereas Social Interaction Support showed the strongest association with Community-Based Social Participation. Age-group differences were non-monotonic and path-specific, while structural heterogeneity was more pronounced across community types. These findings support age-friendly public-space renewal integrating support for use, restorative experiences, and opportunities for social interaction across age groups and community contexts.