Spinel-type MgMn2O4 nanoplates similar to 10 nm thick were prepared as a positive electrode for magnesium rechargeable batteries by the transformation of metal hydroxide nanoplates. Homogeneous coating with a vanadate layer thinner than 3 nm was achieved on the spinel oxide nanoplates via coverage of the precursor and subsequent mild calcination. We found that the spinel oxide nanoplates with the homogeneous coating exhibit improved electrochemical properties, such as discharge potential, capacity, and cyclability, due to the enhanced insertion and extraction of magnesium ions and suppressed decomposition of electrolytes. The nanometric platy morphology of the spinel oxide and the vanadate coating act synergistically for the improvement of the electrochemical performance.