Multi-component nanocrystalline mesoporous metal oxides have attracted considerable attention recently, but their application in energy storage and catalysis has rarely been reported. Herein, nanocrystalline mesoporous NiMn₂O_x materials with ordered mesostructures and high surface areas were prepared and characterized by Powder X-ray Diffraction (PXRD), transmission electron microscopy (TEM), and N₂ adsorption–desorption. The crystal phases and average crystal sizes of mesoporous NiMn₂O_x could be tuned by changing the annealing temperature. Mesoporous NiMn₂O_x annealed at 600 °C has a hematite structure (hexagonal), whereas that annealed at 800 °C has a spinel structure (cubic). The application of mesoporous NiMn₂O_x in electrochemical energy storage and catalytic decomposition of N₂O is also demonstrated.