The in situ CO2 mixing technique is an emerging approach for the permanent sequestration of CO2 during concrete production. While recognized as a promising carbon capture and utilization (CCU) strategy, its effect on the leachability of heavy metals in cementitious materials remains largely unexplored. This study investigates the influence of chemically induced CO2 mineralization on the leaching behavior of hexavalent chromium (Cr(VI)) in cement paste. A tank leaching test revealed that Cr(VI) leaching in CO2-mixed specimens was nearly an order of magnitude lower than in conventionally mixed samples. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) confirmed an increased formation of hydration products in CO2-mixed samples, along with a higher Cr(VI) retention. Thermogravimetric analysis (TGA) indicated significant CrO42− immobilization as CaCrO4, while Fourier Transform Infrared (FTIR) spectroscopy revealed enhanced Cr-O bonding, suggesting structural stabilization. Additionally, microstructural analysis identified CrO42− incorporation within monocarboaluminate hydrates, further contributing to Cr(VI) immobilization. These findings demonstrate that in situ CO2 mixing not only reduces CO2 emissions but also enhances the stabilization of hazardous heavy metals, providing a dual environmental benefit for cement-based materials.