Poly(hexyl viologen dichloride) (PHV-Cl) is described as a high energy density (ED) material in magnesium-based electrolytes. When coupled with Mg metal in all phenyl complex (APC) electrolytes, PHV-Cl cathodes demonstrate anion-transport behaviors with the reduction/oxidation of viologen units. This dual-ion type cell delivers a relatively high ED with reasonable stability during repeated charge/discharge (C/D) when compared with previously reported inorganic/organic cathodes in magnesium-ion batteries (MIBs). PHV-Cl shows a capacity of 171 mAh g−1 at an average discharge potential of 1.33 V vs. Mg/Mg2+, resulting in a significantly high ED (227 mWh g−1). The high ED characteristics of PHV-Cl are retained during repeated C/D cycles (201 mWh g−1 after 50 C/D) in contrast to the relative instability of other previously reported organic cathodes. Herein, the electrochemical performance of PHV-Cl in various types of electrolytes is also described. This work suggests that viologen-based cathodes with anion-transport properties could be implemented to achieve high levels of ED and stability via a dual-ion mode in MIBs, particularly with currently available electrolytes that are compatible with magnesium metal.