Nano-reinforced composites are widely studied by the scientific community. The main factors affecting the final nanocomposite performance are the filler type and content, as well as the duration of the dispersion. In this work, we report the effects of Multi-Walled Carbon Nano Tubes (MWCNTs) and milled Carbon Black (CB) dispersion in epoxy resin on the electrical and mechanical properties of the resulting composites. Impedance Spectroscopy (IS) was utilized to assess the dielectric properties of the specimens. The mechanical properties were evaluated by fracture toughness tests, while Scanning Electron Microscopy (SEM) was performed to study the influence of the reinforcement on the failure mechanisms acting on the fracture surfaces of the specimens. IS results for epoxy/CNT systems revealed the creation of a 3D conductive network for concentrations above 0.3 wt. %, while CB did not result in the formation of such a network for filler contents up to 2 wt. %. However, the synergistic effect of CNTs/CB was successfully manifested by both the optimal electrical properties and the 81% enhanced fracture toughness in comparison to the neat resin. Fractography confirmed the aforementioned results and revealed the fracture mechanisms of all systems, such as crack pinning and deflection, and particle pull-out phenomena.