We report a one-pot synthesis of Mn(MnO)/Mn₅C₂/carbon nanotube (CNTs) nanocomposite for supercapacitors. The Mn(MnO)/Mn₅C₂/CNTs composed of Mn(MnO) nanoflakes bonded with CNTs through interfacial Mn₅C₂ carbides was prepared by heating a mixture of Mn powder and CNTs at 600 °C under vacuum. The carbides obtained by an in situ reaction provided strong interface bonding between the CNTs and Mn(MnO), and consequently enhanced stability of the nanocomposite as a supercapacitor electrode material. The capacitive properties of the Mn(MnO)/Mn₅C₂/CNTs electrodes were investigated by a cyclic voltammetry (CV) test in a 0.5 M Na₂SO₄ aqueous solution. The Mn(MnO)/Mn₅C₂/CNTs prepared at 600 °C for 1 h displayed a maximum specific capacitance of 378.9 F g⁻¹ (based on total mass of active materials) at 2 mV s⁻¹. The long-term cycling stability of the Mn(MnO)/Mn₅C₂/CNT electrode was investigated by repeating the CV test from 0.1 and 0.8 V (vs. SCE) at 100 mV s⁻¹. Contrary to a traditional MnO₂/CNTs electrode, whose specific capacitance would decrease with cycle number, the Mn(MnO)/Mn₅C₂/CNTs had an increased specific capacitance at the initial 450 cycles. This phenomenon is because of an electrochemical conversion from Mn(MnO) to MnO₂ in the initial CV test. Little decrease in the specific capacitance was found even after 1000 cycles, indicating an excellent cycling stability. These properties are attributed to the unique Mn(MnO)/Mn₅C₂/CNT structure.