The changes in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated. Mechanical treatment reduces the average crystallite size of MgFe2O4 to the nanometer range and induces cation redistribution between tetrahedral and octahedral sites. The degree of inversion of the mechanically treated ferrite is compared with that of MgFe2O4 quenched from high temperatures. The range of thermal stability of mechanically induced metastable states is determined by studying the response of the metastable mechanically activated MgFe2O4 to changes in temperature.