Recently, studies have been reported to synergistically improve the electrical conductivity of polymer composites by simultaneously incorporating hybrid fillers, but systematic studies on filler loading and ratio are still scarce. In this study, a one-step process was proposed to induce the incorporation of uniformly dispersed fillers with a high content, and synergistic improvement in the electrical conductivity of polymer composites was studied by applying two types of carbon fillers: nano-sized multi-walled carbon nanotube (MWCNT) and micro-sized pitch-based carbon fiber (PCF). Based on the proposed process, it was possible to fabricate a polymer composite in which the filler was uniformly dispersed within 40 wt%. The electrical conductivity of the composite containing up to 10 wt% MWCNT which was the percolation plateau content and 30 wt% PCF was 3940 S/m, showing the maximum performance. This result was improved by 595% and 586%, respectively, compared to the electrical conductivity of the composite containing only 40 wt% MWCNT or PCF. These findings can contribute to expanding the application of conductive composites in the fields of antistatic or electromagnetic interference shielding by providing insight into the optimal design of hybrid filler systems to improve the electrical conductivity of composites.