Aqueous manganese-ion batteries (AMIBs) are emerging as promising candidates for large-scale, safe, and low-cost energy storage owing to the natural abundance and much lower redox potential of manganese compared with zinc. Here, we realize a high-voltage AMIB by pairing a semi-crystalline, porous polyaniline (PANI) cathode with a Mn metal anode. Exploiting the intrinsically lower Mn potential (-1.19 V vs. SHE) than Zn (-0.76 V) enables an average cell voltage of 1.27 V-over 30% higher than conventional Zn-based aqueous batteries-while keeping PANI within its stable electrochemical window. The PANI cathode delivers a high reversible capacity of 88.3 mAh/g and retains 67.9% capacity after 1000 cycles, with ex situ spectroscopy revealing a highly reversible Mn² ⁺/H⁺ hybrid storage mechanism and exceptional structural stability. Full Mn//PANI cells markedly outperform Zn analogues not only in voltage but also in rate capability and cycle life, maintaining 81.3% capacity after 500 cycles at 1.0 A/g. These findings position PANI as a high-performance organic cathode and establish clear system-level advantages of Mn over Zn for next-generation high-voltage aqueous batteries.