Herein, investigation on XLPE–Al₂O₃–clay ternary hybrid systems of Al₂O₃ and clay in 1 : 1 and 2 : 1 ratios, binary systems of XLPE–clay and XLPE–Al₂O₃ nanocomposites, with special reference to the hybrid filler effect and the superior microstructural development in ternary systems is conducted. The ternary hybrid composite of Al₂O₃ and clay in a 1 : 1 ratio exhibits the highest tensile strength (100% increase) and Young's modulus (208% increase), followed by the Al₂O₃ : clay = 2 : 1 system. The interaction between alumina and clay altered the composite morphology, filler dispersion and gave rise to a unique filler architecture leading to a substantial boost up in mechanics compared to predictions based on the idealized filler morphology. Experimentally observed much higher mechanics compared to theoretical predictions confirmed that the dramatic improvement in mechanics is the outcome of the positive hybrid effect and a second factor of synergism, i.e. filler–filler networks. Morphological control of the hybrid filler network is realized by adjusting the ratio between different fillers. For the Al₂O₃ : clay = 2 : 1 system, the microstructural limitation of dispersion due to the steric effect of alumina clusters shifts the properties to the negative hybrid effect region.