Studies have documented the inconsistent influences of activity participation on health. However, the underlying mechanisms connecting activity patterns to loneliness are relatively understudied. Using data on 8,545 respondents aged 60 or older from the 2014 China Longitudinal Aging Social Survey, we used latent class analysis to identify the patterns of participating in eight activities and path analysis to examine the mediating effects of formal support, informal support, and aging attitudes in the relationship of activity patterns and loneliness. Three meaningful patterns of activity (high activity, low activity, and working caregivers) were identified. Working caregivers and low activity groups, compared to high activity, showed higher levels of loneliness in later life. Informal support, rather than formal support, and attitudes toward aging mediated the relationship between activity patterns and loneliness in later life. This study highlights the importance of fuller engagement in diverse political, economic, and community activities to prevent loneliness. Additionally, intervention programs that facilitate joint support from family, community, and government and shape scientific understanding of aging are critical for alleviating loneliness in later life.