In this paper, in situ nanoparticles (TiB2+ZrB2) and Sb are introduced simultaneously into A356 aluminium alloy to improve the mechanical properties of the material through the synergistic effect arising from the reinforcement of the matrix by the nanoparticles and the enhancement of particle dispersion by Sb. The introduction of (TiB2+ZrB2) particles alone has been shown to refine the α-Al matrix and reduce the secondary dendrite arm spacing. However, the presence of heavy agglomerates in composites can affect their performance. To address this issue, Sb is introduced to reduce the interfacial energy between the nanoparticles and the Al matrix, leading to a significant improvement in the agglomeration phenomenon of the nanoparticles. The synergistic introduction of in situ (TiB2+ZrB2) nanoparticles and Sb results in a substantial increase in the strength and plasticity of the composites compared to those of the A356 matrix. The tensile strength, yield strength, and elongation of the as-cast composites reach 216.4 MPa, 119.7 MPa, and 7.2%, respectively, when the introductions of (TiB2+ZrB2) and Sb are 3 wt. % and 0.6 wt. %, respectively, which are 29.7%, 23.5%, and 84.6% greater than the performance of the A356 matrix, respectively.