Hybrid fillers composed of carbon nanotubes (CNTs) grown on graphene nanoplatelets (GNPs) were dispersed into epoxy matrix to serve as promising reinforcements. And the CNT-GNP/epoxy composite shows distinctive self-sensing behavior for in situ monitoring the onset of irreversibly permanent deformation. Here it has been established that the embedding of CNT-GNP hybrids into pristine epoxy endows optimum dispersion of CNTs and GNPs as well as better interfacial adhesion between the carbon fillers and matrix, which results in a significant improvement in load transfer effectiveness. Remarkably enhanced mechanical properties in the CNT-GNP/epoxy composite were achieved at ultralow hybrid loading (0.5 wt.%). The tensile modulus showed ∼40% increase and the tensile strength was enhanced by ∼36% with respect to the neat epoxy. The reinforcement efficiency of the CNT-GNP hybrids is found to outperform that of the CNT + GNP mixture predicted using the modified Halpin-Tsai modeling. The in situ electrical resistance of the CNT-GNP/epoxy composite initially increases to its maximum value and then begins to decrease with the appearance of residual strain and irreversible deformation, which is remarkably different from the randomly oriented CNTs filled composites only with monotonic increase of the resistance until their catastrophic fracture.