Graphene-related materials (GRM) are promising materials to improve mechanical, electrical, and optical polymer properties. Adding multi-wall carbon nanotubes (MWCNT) on GRM nanocomposites promotes synergistic effects on its electrical properties and electromagnetic interference shielding effectiveness (EMI SE). This study focused on evaluating the addition of MWCNT on three different polycarbonate (PC)/acrylonitrile butadiene styrene (ABS)/GRM nanocomposites. These hybrid nanocomposites were prepared by melt mixing using extrusion and injection-mold processes. The morphologies of the nanocomposites were evaluated by field emission gun scanning electron microscopy (FEG-SEM), and their proprieties were characterized by X-ray diffraction (XRD), oscillatory rheological, dielectric spectroscopy, and EMI SE analyses. The electrical behavior of all hybrid nanocomposites was anisotropic due to the injection-molded preparation process. The lateral size, thickness, and production method of the GRM were indicated to be critical factors for the electrical and EMI SE behaviors of these composites. The nanocomposites with GRM2, which had the smallest lateral size and thickness (graphene nanoplatelets), promoted total attenuation levels around 20 dB with 1.5 wt% of MWCNT, matching the EMI SE required for some commercial applications.