Aluminium matrix composites have remarkable mechanical properties that can satisfy the application requirements of high-performance structural materials in the aerospace field. However, the heat treatment process of aluminium matrix composites mostly follows the process of matrix materials, ignoring the interaction between particle reinforcement and the precipitated phase. In this paper, TiB2 /Al-4.5Cu-1Mn-0.3Mg composites with different TiB2 particle contents (0 wt.%, 1 wt.%, 2 wt.%, and 3 wt.%) were prepared via in situ reactions, and the effects of the TiB2 particle content on the microstructure and mechanical properties of the aluminium-copper matrix composites were investigated. The results show that with increasing TiB2 particle content, the grains of the as-cast composites gradually transform from dendritic crystals to fine isometric crystals. Concurrently, the peak solution temperature required for T6 heat treatment decreases as the solution time and aging time decrease. The quantity and size of the precipitated phase decrease, and the yield strength and elastic modulus of the materials increase, whereas the elongation decreases. Notably, the 3TiB2 /Al-Cu-Mn-Mg composite after T6 treatment has the highest yield strength (467.4 MPa) and ultimate tensile strength (505.3 MPa), surpassing those of the base alloy by 36.9% and 14.4%, respectively.