The present paper demonstrates that multi-scale fillers such as carbon nanofibres (CNFs) and short carbon fibres (SCFs) can significantly improve the mode I fracture toughness of epoxy composites by various toughening mechanisms. A comparative assessment on the toughening performance promoted by CNFs and SCFs is presented along with the effects of aligning the filler normal to the crack growth using an applied alternating current (AC) electric field. For SCF concentrations of up to 1.5 wt%, with a concentration of CNFs of 1.0 wt%, the multi-scale, hybrid reinforcements additively toughen the epoxy polymer, with the measured fracture toughness being up to about fourteen times the value of the unmodified epoxy polymer. When subjected to an external AC electric field, these two reinforcements rapidly align along the direction of the electrical field in epoxy resin, with the CNFs concentrating between the ends of, and depositing on, the SCFs. For the same concentrations of SCFs and CNFs, the electric field induced alignment of the CNFs and the SCFs further increases the fracture toughness of the multi-scale toughened or hybrid epoxy polymer by up to twenty times that of the unmodified epoxy polymer. The intrinsic and extrinsic toughening mechanisms spanning the nano-to-millimetre length scale have been identified, based upon which an analytical model has been proposed.