Abstract In this work, amorphous nickel–cobalt–manganese hydroxide (NiCoMn–OH) was hydrothermally synthesized using a mixed solvent strategy and used as positive electrode materials for supercapacitor-battery hybrid energy storage system. The experimental results show that the mixed solvent is indispensable to form the amorphous phase of NiCoMn–OH, which exhibits significantly improved electrochemical activity and rate capability in comparison with the crystalline counterpart because of more grain boundaries and ion diffusion channels in the former phase. A strong synergy between the transition metal ions in the amorphous NiCoMn–OH is found to significantly contribute to the electrochemical activity, rate capability and cycling stability. In addition to battery behavior, the amorphous NiCoMn–OH exhibits pseudocapacitive behavior, which contributes approximately 40% to the total energy storage capacity. The pseudocapacitive property significantly enhances the rate performance. The robust synthesis method described in this paper was also used to fabricate the NiCoMn–OH porous network on Ni foam, which shows a specific capacity close to its theoretical value, indicating a complete utilization of the electroactive material. Furthermore, a supercapacitor-battery hybrid cell fabricated with the amorphous NiCoMn–OH as the positive electrode and reduced graphene oxide (RGO) as the negative electrode exhibits both high-energy and high-power performances with a specific energy of 42.8 Wh kg–1 at a specific power of 749 W kg–1 or a specific energy of 19.9 Wh kg–1 at a specific power of 20.9 kW kg–1.