<p>This work investigates the influence of molybdenum trioxide (MoO₃) particulates on the mechanical and tribological behavior of an Al-Mg AA5754 aluminum alloy with the objective of overcoming its low hardness and wear resistance. Composites containing 2–8 wt.% MoO₃ were fabricated using ultrasonic-assisted stir casting, which combined mechanical stirring and acoustic cavitation to achieve uniform dispersion, low porosity, and strong particle–matrix bonding. Microstructural observations revealed progressive refinement from coarse dendritic α-Al in the base alloy to interconnected dendritic networks and sub-grain interlocking at 6 wt.% reinforcement, whereas 8 wt.% led to clustering, boundary distortion, and void coalescence. Tensile fractography confirmed this transition: the base alloy failed by coarse micro-void coalescence, 2–4 wt.% exhibited finer dimples with limited interfacial debonding, 6 wt.% showed dense dimples and tear ridges indicative of strong particle–matrix interfaces, while 8 wt.% displayed quasi-cleavage facets from cluster-induced brittleness. Wear surface analysis showed a clear progression in wear mechanisms with increasing MoO₃ content. The base alloy primarily exhibited delamination wear, which transitioned to crack-assisted material removal at 2 wt.% reinforcement. At 4 wt.%, the surfaces revealed a combination of abrasive and delamination wear. The 6 wt.% composite displayed predominantly mild abrasive and oxidative wear with smooth, polished tracks, while the 8 wt.% composite showed abrasive–adhesive wear characterized by particle pull-out and surface pitting. Quantitatively, 6 wt.% MoO₃ achieved optimum properties: tensile strength ~310 MPa, compressive strength ~410 MPa, flexural strength &gt;360 MPa, impact energy ~19 J, hardness ~89 HV, wear rate ~0.4×10⁻⁴ mm³·N⁻¹·m⁻¹, and stable COF ~0.26. These findings confirm that microstructural refinement, fracture mechanisms, and tribo-film stabilization converge at 6 wt.% MoO₃ to deliver superior performance.</p>