Developing highly robust and efficient electrode materials is of critical importance to promoting the energy density of current supercapacitors for commercialization. Herein, we report an efficient catalyst with monodispersed Mn single-atoms embedded in carbon nanotubes (Mn-CNTs) for enhancing the electrode performance of supercapacitors. A high specific capacitance (1523.6 F·g⁻¹ at 1.0 A·g⁻¹) can be achieved, which is about twice as high as the specific capacitance of the electrode material without the introduction of Mn single-atoms. Remarkably, the asymmetric electrochemical capacitor created with Mn-CNT and activated carbon exhibits a high energy density of 180.8 Wh·kg⁻¹ at a power density of 1.4 kW·kg⁻¹, much higher than most reported results. The study shows that the integration of Mn atoms into the CNT can enhance the charge transport capacity and the number of polar active sites of Mn-CNT and then facilitate chemical interactions between Mn-CNT and OH⁻. This work provides a novel strategy to enable high-energy storage in supercapacitors by introducing single-atoms into carbon nanotubes to improve electrodes’ energy density and cycle life.