Syntheses of Mn3O4 involve either high sintering temperatures to produce well crystallized products or the use of water-soluble precursors, surfactants, and organic solvents to generate nanocrystalline products. Mechanochemical approaches are known to be effective in the preparation of fine-grained or nanoscaled materials, while also being environmentally friendly because no solvents and no sintering at high temperatures are required. We report the solvent free mechanochemical synthesis of Mn3O4 nanocrystals at room temperature, from a mixture of MnO and Mn2O3. The single-phase product was characterized by Rietveld refinement and SEM images. The obtained crystallite size was 14.2(2) nm, which is among the smallest ever produced crystallite sizes of Mn3O4. The obtained product reveals an enormous increase in lithium intercalation capability, which was proven via chemical lithium intercalation at room temperature. More than 50% lithiation of nanocrystalline Mn3O4 is observed after a reaction time of 1 h, while coarse-grained material from a solid-state reaction shows no intercalation under the same reaction conditions. Therefore, the produced manganese oxide has a high potential in lithium battery applications.