This study investigates the long-term performance of laboratory dam concrete in different curing environments over ten years and the microstructure of 17-year-old laboratory concrete and actual concrete cores drilled from the Three Gorges Dam. The mechanical properties of the laboratory dam concrete, whether cured in natural or standard environments, continued to improve over time. Furthermore, the laboratory dam concrete exhibited good resistance to diffusion and a refined microstructure after 17 years. However, curing and long-term exposure to the local natural environment reduced the frost resistance. Microstructural analyses of the laboratory concrete samples demonstrated that moderate-heat cement and fine fly ash (FA) particles were almost fully hydrated to form compact microstructures consisting of large quantities of homogeneous C-(A)-S-H gels and a few crystals. No obvious interfacial transition zones were observed in the microstructure owing to the long-term pozzolanic reaction. This dense and homogenous microstructure was the crucial reason for the excellent long-term performance of the dam concrete. A high FA volume also played a significant role in the microstructural densification and performance growth of dam concrete at a later age. The concrete drilled from the dam surface exhibited a loose microstructure with higher microporosity, indicating that concrete directly exposed to the actual service environment suffered degradation caused by water and wind attacks. In this study, both macro performance and microstructural analyses revealed that the application of moderate-heat cement and FA resulted in a dense and homogenous microstructure, which ensured the excellent long-term performance of concrete from the Three Gorges Dam after 17 years. Long-term exposure to an actual service environment may lead to microstructural degradation of the concrete surface. Therefore, the retained long-term dam concrete samples need to be further researched to better understand its microstructural evolution and development of its properties.