Graphene–MWCNT/ABS nanocomposites have been prepared using a solvent-free dry tumble mixing process followed by hot compaction. The combined effect of using two nanofillers (graphene and MWCNTs) on the electrical conductivity and EMI shielding properties of the nanocomposites has been studied. A unique conductive network of MWCNTs and graphene formed due to their different geometrical shapes, besides the high aspect ratios of the MWCNTs. This served to improve the electrical conductivity and the electromagnetic shielding properties. FESEM results for freeze-fractured pellets have confirmed the enhancement in conductivity and shielding of the graphene–MWCNT/ABS nanocomposites and, furthermore, the escape of EM waves through windows/gaps consisting of filler-depleted areas containing ABS. The addition of 1 wt% MWCNTs to a graphene–ABS nanocomposite resulted in a synergistic effect on the EM shielding properties.