Electrochemical characteristics of an aqueous manganese nitrate [Mg(NO₃)₂] electrolyte system have been investigated for electric double layer capacitor (EDLC) applications. Its electrochemical activity has been examined in a concentration range from 0.1 to 1.0 mol dm⁻³ by using a symmetric capacitor based on activated carbon (AC) electrodes. The capacitor performances were evaluated by impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge experiments at temperature 25°C. The observed results show essentially capacitive behaviors within a tested potential region from 0 to 0.8 V vs. an Ag/AgCl reference electrode. The cell capacitance varies with the electrolyte concentration and the measured potential window. The maximum observed capacitance was 124 F g⁻¹ (energy density: 8.5 Wh kg⁻¹). We have confirmed good reversibility of Mg²⁺ adsorption/desorption and the stability of the aqueous electrolyte within the applied potential limit. The results show an ideal capacitor nature with a significant energy density value.