Zeolite-confined silver nanoclusters (Ag-zeolite) have aroused vast interest due to their remarkable luminescence. The countercations within a zeolite play critical roles in determining the luminescent properties of the resulting Ag-zeolite. We observed, in this work, that introducing Mg2+ enabled the Ag-13X zeolite a stable and bright yellow emission with a high PLQY of 94.6%, the first report on the luminescence enhancement of the Ag-13X zeolite by Mg2+, to the best of our knowledge. The formation of specific internal electric fields inside 13X and the structural contraction of the zeolite framework due to the high charge density and the small ionic radius of Mg2+ are believed to be responsible for the enhanced stable and bright yellow emission. The stabilization effect of Mg2+ is removed by increasing the heating temperature above 700 °C, which leads to the variation of silver nanoclusters as a result of the framework collapse of the zeolite. The Ag-zeolite synthesized by us, featured with a broad emission band, a high PLQY of 94.6%, and good thermal stability, can be considered a suitable candidate to replace the traditional commercial yellow-emitting phosphor YAG:Ce3+ for light-based applications. This work contributes to a valuable reference for the rational design of silver nanoclusters confined in zeolites with promising new functionalities and stimulates potential applications as novel phosphors for near-ultraviolet light-emitting diodes (NUV-LEDs).