In this work, we report a facile one-spot synthesis process and the influence of compositional variation on the electrochemical performance of Ni-Mn-oxides (Ni:Mn = 1:1, 1:2, 1:3, and 1:4) for high-performance advanced energy storage applications. The crystalline structure and the morphology of these synthesized nanocomposites have been demonstrated using X-ray diffraction, field emission scanning electron microscopy, and transmission electron Microscopy. Among these materials, Ni-Mn-oxide with Ni:Mn = 1:3 possesses a large Brunauer-Emmett-Teller specific surface area (127 m(2) g(-1)) with pore size 8.2 nm and exhibits the highest specific capacitance of 1215.5 F g(-1) at a scan rate 2 mV s(-1) with an excellent long-term cycling stability (similar to 87.2% capacitance retention at 10 A g(-1) over 5000 cycles). This work also gives a comparison and explains the influence of different compositional ratios on the electrochemical properties of Ni-Mn-oxides. To demonstrate the possibility of commercial application, an asymmetric supercapacitor device has been constructed by using Ni-Mn-oxide (Ni:Mn = 1:3) as a positive electrode and activated carbon (AC) as a negative electrode. This battery-like device achieves a maximum energy density of 132.3 W h kg(-1) at a power density of 1651 W kg(-1) and excellent coulombic efficiency of 97% over 3000 cycles at 10 A g(-1).