<p>The cognitive function of middle-aged and elderly people is associated with individual and family well-being, providing a crucial foundation for national development plans to promote healthy and active aging. Internet development has brought about significant transformations in their lives. Previous research examining the link of the Internet and cognitive function has mainly focused on Internet use levels and favored correlation analysis, lacking empirical evidence reflecting causality. This study interprets Internet development from a policy perspective, using the 2014–2016 “Broadband China” strategy as a quasi-natural experiment, which is then innovatively introduced into cognitive function research. Using the 2011–2018 China Health and Retirement Longitudinal Study (CHARLS) data, we empirically examined the causal effect of Internet development on middle-aged and elderly people’s cognitive function and its mechanism by employing the time-varying difference-in-differences (DID) and mediation effect models. Results suggest that Internet development can effectively improve their cognitive function. Specifically, the implementation of the “Broadband China” strategy has led to an average increase of 2.45% in cognitive function levels. These results remain consistent following a series of robustness tests. Internet development can improve cognitive function by encouraging a healthy lifestyle (social interaction and physical exercise), relieving depressed feelings, and enhancing physical health. Among them, the mechanism explanation of physical exercise accounts for the highest proportion at 7.28%. Furthermore, at the regional level, this positive impact is more noticeable in inland provinces and rural areas. At the individual level, the elderly and male groups’ cognitive function received more pronounced enhancement effects from Internet development. This systematic evidence of causality contributes to recognizing the Internet’s importance in safeguarding this population’s cognitive health and provides a decision-making reference for governments to adopt corresponding measures to improve their cognitive function. </p>