The rational design of earth-abundant catalysts with excellent water splitting activities is important to obtain clean fuels for sustainable energy devices. In this study, mixed transition metal oxide nanoparticles encapsulated in nitrogendoped carbon (denoted as AB₂O₄@NC) were developed using a one-pot protocol, wherein a metal–organic complex was adopted as the precursor. As a proof of concept, MnCo₂O₄@NC was used as an electrocatalyst for water oxidation, and demonstrated an outstanding electrocatalytic activity with low overpotential to achieve a current density of 10 mA·cm^-1 (η₁₀ = 287 mV), small Tafel slope (55 mV·dec^-1), and high stability (96% retention after 20 h). The excellent electrochemical performance benefited from the synergistic effects of the MnCo₂O₄ nanoparticles and nitrogen-doped carbon, as well as the assembled mesoporous nanowire structure. Finally, a highly stable all-solid-state supercapacitor based on MnCo₂O₄@NC was demonstrated (1.5% decay after 10, 000 cycles).