Maraging stainless steel is a high-strength material with excellent corrosion resistance and weldability, making it widely used in the aviation, aerospace, marine, and energy sectors. This study systematically investigates the microstructural evolution and tensile behavior of a Cu-containing maraging stainless steel across different aging temperatures to overcome the strength–ductility trade-off and elucidate the role of Cu in microstructural tuning and tensile property optimization. The thermodynamic mechanism of reversed austenite formation was analyzed based on the double-spherical-cap model. In addition, the contributions of various strengthening mechanisms to yield strength evolution were quantitatively evaluated. The results demonstrate that as aging temperature increases from 480 °C to 560 °C, the number densities of fcc Cu-rich precipitates and Laves-phase Fe₂Mo type Mo-rich phases exhibit a nonmonotonic trend, first increasing and peaking at 500 °C, then decreasing, while their sizes increase continuously with temperature. Higher aging temperatures accelerate reversed austenite formation and slightly increases the effective grain size and reduce the density of geometrically necessary dislocations. Furthermore, tensile properties exhibit a nonmonotonic response, yield strength first increases and then decreases with increasing aging temperature, whereas elongation improves continuously across the full temperature range.