Nanocomposites of poly­(methyl methacrylate)/reduced graphene oxide (PMMA/rGO) without and with decorated magnetite nanoparticles with a segregated structure were prepared using emulsifier-free emulsion polymerization. Various characterization techniques were employed to validate the presence of the nanofillers and the formation of the segregated structure within the nanocomposites. The percolation threshold of the nanocomposites was found to be 0.3 vol %, while a maximum electrical conductivity of 91.2 S·m–1 and electromagnetic interference shielding effectiveness (EMI SE) of 63.2 dB (2.9 mm thickness) were achieved for the PMMA/rGO nanocomposites at a loading of 2.6 vol % rGO. It was also observed that decorating rGO with magnetite nanoparticles (hybrid nanocomposites) led to a tremendous increase in EMI SE. For instance, 1.1 vol % PMMA/rGO nanocomposites indicated an EMI SE of 20.7 dB, while adding 0.5 vol % magnetite nanoparticles enhanced EMI SE to 29.3 dB. The excellent electrical properties obtained for these nanocomposites were ascribed to both superiorities of the segregated conductive structure and magnetic properties of the magnetite nanoparticles.