Abstract: To improve the densification, strength, and toughness of polycrystalline cubic boron nitride (PcBN) composites, cBN particles with four particle-size ranges—0.5–1.0 μm, 1.0–2.0 μm, 2.0–4.0 μm, and 4.0–6.0 μm—were used as starting materials. An Al–Ti–Al2O3 multicomponent binder was employed, and different amounts of onion-like carbon (OLC) were introduced to tailor interfacial bonding. PcBN composites were fabricated under high-pressure and high-temperature conditions of 5 GPa and 1500 °C, followed by systematic characterization of their phase composition, microstructure, and mechanical and wear-resistant properties. The results showed that an appropriate amount of OLC markedly improved the microstructural dispersion of the binder phase and provided abundant heterogeneous nucleation sites for the precipitation of ceramic phases. The relative density first increased and then decreased with increasing OLC content, reaching a maximum of 99.5% (ρ = 3.508 g·cm-3) at 0.5 wt% OLC. The flexural strength reached its highest value of 624 MPa at 1.0 wt% OLC. When the OLC content was 2.0 wt%, the hardness, fracture toughness, and wear ratio reached their peak values of 3814.8 HV, 9.0 MPa·m1/2, and 6425, respectively. Therefore, when hardness, fracture toughness, and wear resistance are considered the primary evaluation criteria, the composite containing 2.0 wt% OLC exhibits the best overall performance, whereas the maximum relative density and flexural strength are achieved at lower OLC contents.