Nonstoichiometric TiC_0.5 and (TiNb)_0.5 powders were prepared by the mechanical alloying process using Ti, Nb, and TiC powders as raw materials. Furthermore, the as-prepared TiC_0.5 and (TiNb)_0.5 powders were used as initial materials to fabricate TiC_0.5 and (TiNb)_0.5 compacts under high pressures and high temperatures (HTHP) of 5.5 GPa and 1200–1550 °C for 5 min. Phase identification and microstructure of the mechanical-alloyed powders and the sintered TiC_0.5 and (TiNb)_0.5 compacts were realized by an X-ray diffractometer and scanning electron microscope. The results indicate that the as-prepared TiC_0.5 and (TiNb)_0.5 powders have a similar crystal structure of face-centered cubic (FCC) to TiC. The sintered (TiNb)_0.5 compact has good Vickers hardness (~16 GPa), and notably, excellent fracture toughness (~7.3 MPa·m^1/2). The non-stoichiometric compound not only reduced the sintering temperature of covalent compounds, but also greatly enhanced the mechanical properties of the materials. Thus, we have provided a novel synthetic strategy for the production of a compound with high-strength covalent bonds.