The influence of mechanical activation of mixtures of MnO2 with LiOH and Li2CO3, combined with the following short thermal treatment at different temperatures, on the structure, the processes of disordering, oxygen stoichiometry and dispersion of lithium–manganese spinel has been investigated. It was shown that the mechanochemically prepared compounds are characterized by high dispersion, strongly compressed lattice, the increased amount of Mn (4+) ions and two types of disordering (cation vacancies and cation mixing). The amount of defects and dispersion of product decreases and the crystallinity increases with the temperature of the following thermal treatment. The nature of reagents and the conditions of mechanical activation (MA) influence on parameters of spinel, especially at moderate temperatures.