This study investigated psychological mechanisms linking older adults’ internet use and cognition, exploring age and gender differences. Using data from 2,064 participants in the 2012, 2014, and 2016 waves of the Health and Retirement Study, internet use was measured by access and engagement in online activities, including informational use, social use, online shopping, and online banking. Path analyses tested the mediating roles of loneliness, depressive symptoms, and perceived control, and multi-group analyses examined age and gender differences. Results showed internet use was positively associated with cognition, with a large direct effect and small but significant indirect effects via depressive symptoms and perceived control across all online activities. Multi-group analyses revealed age and gender differences: depressive symptoms mediated the effects of all online activities on cognition among young-old adults and older women, as well as most activities (except online banking) among older men. In contrast, perceived control mediated these effects among old-old adults and the associations between most online activities (except social use) and cognition among older men. Notably, none of the indirect effects were significant when analyses were restricted to internet users. These findings highlight depressive symptoms and perceived control as key mediators in the internet use-cognition association, alongside age and gender differences. Internet-based cognitive interventions should address these mediators, demographic variations, and user status to optimize outcomes for diverse older adults.