Magnesium metal complex oxides are potential electrode materials for magnesium ion batteries with high specific capacities. However, the strong electrostatic interaction between Mg2+ and the host lattice due to its divalency induces slow intercalation kinetics of Mg ions within the crystal lattices. Thus, nanocrystalline particles with shortened Mg ion diffusion distance enable the insertion/extraction of Mg ions and improve the specific capacities of the batteries. Herein, we report the facile rapid production of crystalline MgCo2O4 and Mg2/3Ni4/3O2 nanocrystals by rapid supercritical fluid processing. The phase transition from spinel to rocksalt during the Mg2+ ion intercalation has been confirmed by high-resolution transmission electron microscopy. The nanosheets of Mg2/3Ni4/3O2 rocksalt nanocrystals were controllably synthesized for the first time, which are active materials for magnesium-ion batteries.