Mn and Li were selectively extracted from the manganese-rich slag by sulfation roasting-water leaching. The extraction mechanisms of Mn and Li were investigated by means of XRD, TG-DSC, and SEM-EDS. 73.71% Mn and 73.28% Li were leached under optimal experimental conditions: acid concentration of 82 wt.%, acid-to-slag mass ratio of 1.5:1, roasting temperature of 800 °C, and roasting time of 2 h. During the roasting process, the manganese-rich slag first reacted with concentrated sulfuric acid, producing MnSO₄, MnSO₄·H₂O, Li₂Mg(SO₄)₂, Al₂(SO₄)₃, and H₄SiO₄. With the roasting temperature increasing, H₄SiO₄ and Al₂(SO₄)₃ decomposed successively, resulting in generation of mullite and spinel. The mullite formation aided in decreasing the leaching efficiencies of Al and Si, while increasing the Li leaching efficiency. The formation of spinel, however, decreased the leaching efficiencies of Mn and Li.