Trimanganese tetraoxide (Mn3O4) is limited in supercapacitor application due to its poor electrical conductivity and cycle stability. An effective strategy for improving its electrochemical performance is to be combined with good conductive materials, such as carbon materials. A facile method was developed to prepare a Mn3O4/activated carbonitride composite (MONC) as electrode material for supercapacitor. Mn3O4 particles with small size were homogeneously grown on the surface of activated carbonitride (NC). Notably, the addition of NC not only improves the electrical conductivity of Mn3O4 but also serves as a supporting matrix to maintain the stability of the composite. Electrochemical characterization results show that the specific capacitance of the composite can reach 180 F/g at a current density of 0.5 A/g, which is two times higher than that of Mn3O4 at the same current density. After 2000 cycles, the specific capacitance of MONC can be maintained at 80.2% of the initial specific capacitance. The symmetric coin cell (SCC) assembled by MONC as positive and negative electrodes shows large voltage window, excellent cycle stability, and superior energy/power densities. This work will be one of important references for the application of other transition metal oxides in energy storage devices.