X-ray diffraction and scanning electron microscopy methods were used to study the effect of the planetary ball mill treatment time on the morphology, phase composition and microstructure of the Al–Mn–Cu-based alloy granules with and without nanodiamond particles. The phase composition of the alloy was determined by X-ray diffraction after casting and milling for 5–20 h. It was shown that nanodiamond particles promote granule coarsening, and this is especially noticeable with an increase in the milling time up to 20 h. At the same time, the size of initial alloy granules weakly depends on the processing time. Cu-bearing phases of crystallization origin dissolve during mechanical alloying. The lattice constant of the aluminum solid solution decreases after 5-hour treatment to 0.4028–0.4030 nm, and increases with further increasing milling time. Exothermic effects associated with the precipitation of secondary phases were revealed for mechanically alloyed granules during heating. An increase in the milling time reduces the intensity of peaks. The solidus temperature of samples decreased after mechanical alloying. For the nanodiamond-bearing sample, an exothermic effect is observed which can be ascribed to the aluminum carbide formation or oxidation reactions in nanodiamond particles. The maximum microhardness is achieved after 5–10 h of mechanical alloying, and the nanodiamond particles slightly increase the maximum microhardness from 316 to 330 HV. The results indicate the dissolution of copper and manganese in the aluminum solid solution after 5 h of treatment and their precipitation with the increasing milling time. Nanodiamond particles have no effect on the dissolution of elements but accelerate the solid solution decomposition with the increasing treatment time.