Abstract: Mine backfilling is a critical geotechnical operation for underground stability and waste management, while its environmental performance is increasingly constrained by the high carbon footprint of cementitious binders, particularly ordinary Portland cement (OPC). This study investigates the use of CO₂-mineralized steel slag as a reactive supplementary binder for mine backfill to develop a carbon-reducing mine backfill (CRMB), while simultaneously enhancing mechanical performance and reducing carbon emissions. The results show that moderate carbonation (~50% carbonation degree) significantly improves backfill performance, with CRMB2 achieving a 24.5% increase in 28 d compressive strength compared with the uncarbonated system and slightly outperforming OPC under identical conditions. Mechanistic analyses demonstrate that CO₂ mineralization induces the formation of highly reactive CaCO₃ and silica gels, which reprogram hydration pathways by accelerating silicate and aluminate reactions. Furthermore, the availability of carbonate species promotes the formation of stable carboaluminate phases, contributing to sustained strength development at later ages. Nanoindentation also confirms that both low- and high-density C–S–H phases exhibit enhanced stiffness at moderate carbonation levels. From an environmental perspective, CRMB reduces embodied CO₂ emissions by up to ~60% relative to OPC, resulting in superior strength-to-emission efficiency and demonstrating its effectiveness as a carbon-sequestering supplementary binder for mine backfill.