Owing to their excellent mechanical and electrical properties, 2D-3D composites have attracted extensive attention. Their novel 2D-3D structure challenges the prevailing theoretical understanding of the mechanical behaviours of composites. In this study, we proposed a new model for the theoretical prediction of the Young's modulus, fracture strength, and fracture toughness of 2D-3D composites. We accurately predicted the mechanical properties of a typical 2D-3D composite, namely graphene-diamond (Gradia), by introducing elastic and plastic buckling correction factors to modify the rule-of-mixtures and Griffith criterion, respectively. Furthermore, we found that Gradia composites exhibit a combination of the high strength (80-316 GPa) of diamond and excellent fracture toughness (4-6 MPa center dot m(1/2)) of graphene, where the excellent toughness results from the formation of long-chain structures in graphene and amorphisation near the interface. These deformational behaviours enable Gradia to withstand large strains and absorb more energy before structural collapse, resulting in superior toughness. Our work plays an important role in providing a theoretical basis of the mechanical properties of novel 2D-3D composites; moreover, it provides mechanistic insights into the strengthening and toughening mechanisms in Gradia composites.