A series of copper manganese oxide catalysts have been prepared by mechanochemical grinding of carbonate precursors and tested for the low temperature oxidation of CO. It is shown that a 72 h grind time and heat treatment under oxygen is necessary to form active CuMn₂O₄ for the conversion of CO to CO₂ at room temperature. Energy dispersive X-ray analysis mapping indicated that the degree of mixing was improved at longer grinding times. Contraction of the MnCO₃ lattice, followed by X-ray diffraction, showed that grinding times in excess of 24 h were necessary for the near complete dissolution of Cu ions into the MnCO₃ framework, which in turn facilitated the formation of CuMn₂O₄ upon calcination. This simple, clean and environmentally friendly synthesis shows how mixed metal oxides that are catalytically active at room temperature can be synthesized without employing aqueous precipitation techniques, which result in waste streams containing carbonate and nitrate contaminants.