The properties and structure of magnetic graphene oxide GrO/iron oxide FeOx nanocomposites synthesized by the mechanochemical method with different content of components GrO : FeOx (wt %), namely, 20 : 80, 50 : 50, and 80 : 20 were studied. The method of mechanochemical synthesis is a mechanical process of grinding iron oxide powder together with graphene oxide in a ball mill in an aqueous medium. The synthesized magnetic GrO/FeOx nanocomposites were studied by Raman spectroscopy, a vibrating sample magnetometer, and Mössbauer spectroscopy. The Mössbauer studies made it possible to determine the phase composition and structure of the synthesized magnetic GrO/FeOx nanocomposites. The data of Mössbauer spectroscopy showed that the GrO/FeOx composites consist of the magnetite phase Fe3O4 and magnetic nanoparticles in the paramagnetic state, which is consistent with the data of X-ray diffraction studies. Based on the results of Mössbauer spectroscopy, it was found that, in addition to magnetite, the magnetic GrO/FeOx nanocomposites contain hematite α-Fe2O3, as well as phases identified as iron carbides and irondepleted carbon clusters. The latter were not detected by X-ray diffraction, apparently because their number is insignificant and they are in an amorphous state. The results obtained show that graphene is not just a source of carbon during grinding in a ball mill, but has its own reactivity and the ability to generate new phases during mechanochemical activation. Based on the performed Mössbauer spectral studies, we obtained unique and important information on the magnetic structure of the magnetic GrO/FeOx nanocomposites. The research results make it possible to explain the magnetic properties of magnetic nanocomposites, GrO/magnetic particles, which is important for the development and graphene oxide-based synthesis of high-performance magnetic nanocomposites for various applications, including biomedicine.