Dual-electron transfer with Mg2+-ion intercalation outperforms typical alkali metal-ion (Li+, Na+, K+) systems with superior charge storage efficiency while the neutral electrolytes can achieve a working voltage beyond the hydrolysis window of 1.23 V. Hence, aqueous Mg-ion electrolytes are promising for electrochemical energy storage devices to boost the energy density and solve the safety challenges synchronously. However, the Mg-based electrochemical energy storage (EES) devices are generally confined by poor rate performance due to the slow Mg2+ diffusion in the electrode materials. In this paper, we demonstrate that carbon-deficient carbide could function as a promising electrode material in Mg2+-ion-based EES. An electrode made of such carbide can operate over an extended window up to 2.4 V in 1 M magnesium acetate, showing superior performance of high capacitance (125.2 F/g), high energy density (25.1 Wh/kg), and high power density (3934.8 W/kg). Ab initio simulation reveals migration energy of Mg2+ being lower than that of Li+ diffusing from one carbon defect to another in the α-MoC1-x lattice, supporting the experimental results that a symmetric supercapacitor made of α-MoC1-x in an electrolyte based on Mg2+ outperforms electrolytes based on Li+.