In this study, the feasibility of using red mud (RM)-loess (L) mixture (RM-L) to treat acidic wastewater (AW) containing Mn(II) was investigated through batch experiments. The optimum mass ratio of RM to L was obtained by exploring the treatment performance of varied mass ratios of RM to L on AW under different initial pH values. Furthermore, the effects of dosage, contact time, and initial concentration on the treatment of AW with RM-L under optimum mass ratio were explored. The removal mechanism of Mn(II) was clarified using adsorption mathematical models combined with characterization. The results indicate that the optimum mass ratio of RM to L was 7:3. The addition of L promotes the equilibrium adsorption for Mn(II) on RM-L under optimum mass ratio (RM-L (7:3)) and improves the dispersibility of RM-L (7:3). Moreover, hydroxyl, carbonate, and metal-oxygen groups in RM-L (7:3) play an important role in the removal of Mn(II) and the increase in pH value to 8.4 from 3. The adsorption type of Mn(II) is monolayer adsorption, and the adsorption process of Mn(II) is spontaneous and dominated by chemical adsorption.