Coal-based reduction roasting is a widely used processing unit for recycling valuable metals (Li, Ni, Co, and Mn) from Mn-rich spent NCM black mass. However, solid carbon reductants require high temperatures (≥650 °C) and promote the formation of water-insoluble LiAlO₂ via reactions between Li₂O and Al-/F-bearing impurities. Herein, Mn-rich spent NCM black mass after mild NaOH leaching (1 wt%) was subjected to self-reduction, CO reduction, and H₂ reduction, and the three routes were systematically compared. Thermodynamic calculations, XRD, and XPS were used to elucidate phase evolution and element migration. Among the three routes, CO reduction achieved the highest lithium recovery of 87.4% (relative to the lithium remaining in the NaOH-pretreated alkaline leaching residue; the lithium originally present in the Mn-rich spent NCM black mass is the denominator for the overall mass-balance basis) at 10% CO, 600 °C and 90 min, outperforming self-reduction (60.4%) and H₂ reduction (60.7%). The superior performance of CO reduction is attributed to a synergistic effect between externally supplied CO and residual organics/graphite, which generates a controlled atmosphere that promotes Li₂CO₃ formation while suppressing LiAlO₂ crystallization. Subsequent sulfuric acid leaching of the water-leaching residue enabled nearly complete recovery of Ni, Co, and Mn, with leaching efficiencies exceeding 99.9%. This work provides mechanistic insight and comparative process data for optimizing reduction roasting–water leaching routes for selective metal recovery from Mn-rich spent NCM black mass.