Redox-mediated electrodialysis (redox-ED) is one of the most advanced electrochemical ion removal technologies, offering continuous and efficient ion removal. Although advantageous over previous systems, comprehensive assessments of its ion removal performance and the influence of operational parameters are scarce. This study optimized a scaled-up redox-ED cell for the removal of calcium (Ca²⁺) and magnesium (Mg²⁺) ions, targeting water softening applications, and evaluated its electrochemical performance. Under optimal conditions, the system achieved a charge efficiency exceeding 75% and a total hardness removal rate of over 95%, with reasonable energy consumption. Specifically, the removal rates for Ca²⁺ and Mg²⁺ were 90.5% and 90.8%, respectively, demonstrating the system’s feasibility for use in water softening modules. Key factors affecting performance were scrutinized, revealing increased internal resistance as a significant hurdle. Furthermore, the integration of conductive ion exchange resins within the feed channel notably improved Ca²⁺ and Mg²⁺ removal rates, underscoring the potential of the redox-ED system in water softening applications.