In an era characterized by both rapid development in information technology and population ageing, internet use has become a significant factor influencing the mental health of middle-aged and older adults. Using the nationally representative data, this study examines the mechanisms and age heterogeneity of internet use behaviors, including internet access, duration of use, and usage types, on mental health outcomes. This study found that using the Internet can reduce depression tendencies among middle-aged and older adults, with the expansion of social relationships serving as a mediating mechanism. Furthermore, the positive impact of longer internet use duration on mental health is achieved through the expansion of social relationships. Usage types exhibit divergent effects. Video entertainment type is associated with lower depression tendencies. In-depth informational use leads to a higher risk of depression, though social relationships mitigate this negative effect. Additionally, the impact of different Internet use behaviors on depressive tendencies varies among different age groups. The results provided empirical support for developing targeted mental health intervention strategies and optimizing the mental health service system under the Internet environment.