Epoxidation is an efficient chemical process for manufacturing styrene oxide; however, it remains a huge challenge to develop cost-effective and environment-friendly catalysts for facilitating this process. Herein, we reported a single-atomic-site Ag catalyst supported by mesoporous graphitic carbon nitride (Ag1/mpg-C3N4) with an Ag loading of up to 10.21 wt %, which is the highest loading among single-atomic-site Ag catalysts. The single-atomic-site Ag replaced the nitrogen of mpg-C3N4, possessing the Ag1–C2N1 structure. The Ag1/mpg-C3N4 catalyst with the Ag1–C2N1 structure exhibited outstanding catalytic performance for styrene epoxidation with excellent conversion (96%) and high selectivity (81%). This performance was superior to that of the analogous Ag nanoparticle/mpg-C3N4 catalyst and previously reported catalysts. Density functional theory calculations showed that the absorbed superoxide-like O2 species and the much lower reaction barrier to generating styrene oxide than the other products lead to high activity and selectivity, respectively.