The influence of dispersion/mixture time on microstructural evolution and mechanical properties of aluminum matrix nanocomposites reinforced by carbon nanotubes (Al–CNTs) were investigated in this study. The nanocomposites were produced by conventional powder metallurgy routes. The microstructure was characterized by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM). Microstructural analysis shows clusters of carbon nanotubes (CNTs) mainly at the grain junctions, but also CNTs well dispersed and embedded in the aluminum matrix. A strong strengthening effect of the CNTs on the aluminum matrix was measured for the nanocomposites produced with 15 min of dispersion/mixture time. For shorter times than 15 min, it is not possible to obtain a dispersion, which can lead to a reinforcement of the aluminum matrix, while longer dispersion/mixture times cause a decrease in mechanical strength due to the damage caused by the increase in the number of defects and junctions of CNTs.