Polymer composites are widely employed in engineering and industrial applications owing to their tunable properties, lightweight structure, and enhanced performance achieved through the incorporation of fillers and additives. High-density polyethylene (HDPE) composites are of particular interest due to their chemical resistance and mechanical strength; however, their non-polar nature often results in weak interfacial bonding with polar additives. This challenge highlights the need for systematic investigations of filler-matrix interactions. This study examines the mechanical and electrical performance of HDPE-based composites incorporating calcite, graphite, ethyl vinyl acetate (EVA), elastomer, and crosslinker. The significance of this work lies in its multi-factorial approach, which evaluates the synergistic effects of multiple additives rather than relying on single-variable analyses. A total of twenty-six HDPE composite samples were fabricated using extrusion and injection molding processes designed through a statistical experimental plan. Samples were characterized by tensile strength, hardness, and electrical conductivity tests, alongside surface morphology analyses (SEM-EDS). Statistical evaluation was performed using analysis of variance (ANOVA) and response surface methodology (RSM) to determine the significance of parameters and their interactions. Results revealed that yield strength was best explained by the second-order model (quadratic), hardness by the first-order model (linear), while electrical conductivity did not fit the tested models. SEM-EDS further indicated poor dispersion and weak interfacial bonding between the HDPE matrix and additives. In conclusion, the findings emphasize the critical influence of additive type and proportion on the performance of HDPE composites.