Aluminum (Al)-based composites reinforced with graphene and fly ash are promising materials due to their low density and high hardness. One of the critical factors in achieving optimal mechanical properties is the powder preparation stage prior to the sintering process. This study aims to optimize the powder preparation parameters of Al/graphene/fly ash composites with respect to density and hardness using the Taguchi method. The process parameters investigated include reinforcement contents of 5%, 10%, and 15%, milling times of 2 h, 4 h, and 6 h, sintering temperatures of 500 °C, 550 °C, and 600 °C, and holding times of 0.5 h, 1 h, and 1.5 h. The experimental design was conducted using a Taguchi orthogonal array to determine the most influential parameter combinations. The results indicate that increasing the graphene/fly ash reinforcement fraction enhances hardness but reduces density due to increased material porosity. Milling time significantly contributes to the homogeneity of powder dispersion, while sintering temperature and holding time act as dominant factors in interparticle bonding and final densification of the composite. Optimization based on the signal-to-noise (S/N) ratio reveals that the optimal parameter combination for simultaneously maximizing density and hardness consists of a reinforcement content of 10%, a milling time of 4 h, a sintering temperature of 600 °C, and a holding time of 1 h. Experimental validation confirms an increase in density by 7.8% and hardness by 11.4% compared to the initial, non-optimized condition. This study demonstrates that the Taguchi method is effective in identifying optimal powder preparation parameters in the fabrication of Al/graphene/fly ash composites to enhance density and hardness. These findings provide a valuable reference for the development of lightweight, high-performance composite materials for engineering applications.