This paper elucidates the effect of silicon carbide nanoparticles (SiCNP) and graphene nanoplatelets (GNPs), on their own and together, on the densification behavior and fracture toughness of alumina (Al2O3) ceramic matrix. This was investigated by using the high-frequency induction heat sintering (HFIHS) process. While the addition of each nanostructure caused varying degrees of grain refinement and enhancement of mechanical properties, the incorporation of as little as 0.5 wt.% GNPs along with 5.0 wt.% SiCNP promoted uniform dispersion of the latter due to the lateral surface area of the graphene nanosheets with their two-dimensional morphology. There was an associated reduction in grain size from 1500 to 300 nm upon the addition of both types of nanoscale reinforcements. Extensive electron microscopy of the as-produced nanocomposites indicated the presence of SiCNP within, as well as at, the grain boundary areas whereas the 2D GNPs anchored between neighboring grains. Fractography of the samples revealed a transition from a mixed intergranular/transgranular mode for SiCNP or GNP-reinforced nanocomposites to transgranular fracture mode for the hybrid nanocomposites with improvements in fracture toughness and microhardness by 160 and 27%, respectively, largely due to the synergic role of the nanostructured reinforcements and their distinctly different toughening mechanisms. A new toughening model is proposed for the hybrid nanocomposites by taking into consideration crack deflection and pull-out effects due to SiCNP and the atomic level slip-stick driven GNPs inter-layer slithering. It was found that the addition of GNPs facilitates SiCNP dispersion that subsequently develops dense, fine-grained microstructures after a short-cycle, pressure-assisted consolidation process.