Magnesium-ion (Mg²⁺) batteries (MIB) can be desirable relative to lithium ion batteries (LIB) as they offer the opportunity for high volumetric capacity due to the divalent nature of Mg²⁺ (3832 mAh/cm³ of Mg) and enhanced safety due to unlikely dendrite formation on Mg anodes. Spinel magnesium manganese oxide (MgMn₂O₄) has a tetragonal structure different from the typical cubic structure of spinels, due to distortion induced by the strong Jahn-Teller effect of Mn³⁺. In this work we present the first systematic study of MgMn₂O₄ as a cathode material in MIB. The materials were synthesized by a modified Pechini method, and annealing temperature was used to control the crystallite size of the products. The electrochemistry of two different sized samples was studied in magnesium based electrolyte with and without added water in the electrolyte. The MgMn₂O₄ samples were characterized by synchrotron X-ray powder diffraction (XPD), Transmission electron microscopy (TEM) data was collected and the BET surface area was determined. Capacity retention and rate capability was determined in the presence and absence of water in the electrolyte for both samples. Results with respect to crystallite size and water content will be discussed.