The microstructure, phase composition and hardness evolutions of high-energy ball milled Al-15.3%Mn-6.2%Cu alloy and alloy matrix composites with 2% and 5% of Al₂O₃ fine particles were studied with scanning electron microscopy, X-ray diffraction, and differential thermal analysis. Milling provided for the dissolution of the Cu-bearing non-equilibrium CuAl₂ and equilibrium Al₂₀Cu₂Mn₃ phases of solidification origin. The minimum values of the aluminum solid solution lattice parameter of 0.4028 nm suggested 4.4 wt%Mn and 6.2 wt%Cu solute content in Al after milling for ∼5 h, this milling duration providing for the lowest microhardness. Solid solution decomposition with the precipitation of the Mn-enriched phase and precipitation hardening occurred with a further increase in milling time and during annealing of the pre-milled samples. The Al₂O₃ particles refined the mechanically alloyed granules, increased the microhardness and facilitated both the dissolution of the solidification origin phases and the precipitation of the Mn-enriched phase during mechanical alloying, and also improved the precipitation hardening effect.