The three-dimensional microporous framework of the oxide Mo2.5+yVO9+δ (containing Mo5+/6+ and V4+/5+) is defined by three-, six-, and seven-membered ring channels. Due to the oxidation–reduction properties of Mo and V ions and the large open tunnels that can provide a diffusion pathway for small ions, Mo2.5+yVO9+δ has been found to intercalate both Li+ ions, reported previously, and also Mg2+ ions in Mg-ion batteries. Cathodes composed of Mo2.5+yVO9+δ were discharged and charged in Mg-ion cells at current densities ranging from 2 mA/g (C/70) to 10 mA/g (C/12). Mg2+ ions can be inserted into and extracted from MgxMo2.5+yVO9+δ between ∼3.33 and 1.73 V vs Mg/Mg2+ at room temperature, with up to 3.49 Mg2+ ions per formula unit intercalated into the framework, corresponding to a capacity of 397 mAh/g (1st discharge) which is among the highest capacities reported for Mg-based batteries. Powder X-ray diffraction was used to determine the lattice parameters of MgxMo2.5+yVO9+δ (0 < x ≤ 3) compositions prepared by chemical insertion using (C4H9)2Mg. Our findings suggest that in order to overcome the significant shortcomings observed in the traditional Mg-ion Chevrel cathodes (low operating voltage and specific capacity), oxide-based intercalation cathodes can be used instead of the chalcogenides. The slow Mg diffusion kinetics caused by strong Coulombic interactions of the multivalent cations are offset by using a microporous oxide to allow easy migration and minimize steric effects. The combination of Mo and V ions, which can change oxidation state by more than one, can reduce volume expansion and help with charge redistribution during insertion of Mg2+ ions to maintain electroneutrality. This approach based on a microporous molybdenum–vanadium oxide suggests the possibility of using other porous mixed metal oxides for the development of advanced secondary multivalent-ion batteries.