In this study, the effects of heat treatment, friction stir processing (FSP), and the addition of nano-sized SiO₂ powder on the mechanical and microstructural properties of AZ91C magnesium alloy were investigated. The primary aim was to experimentally evaluate the influence of the number of FSP passes and the application of nanoparticle reinforcement in order to identify effective processing conditions. The FSP parameters including a rotational speed of 1250 rpm, traverse speed of 40 mm/min, and tool tilt angle of 3° were kept constant. A dual-tool FSP approach was employed to ensure uniform powder distribution, followed by a T6 heat treatment based on ASTM B661-99. Microstructural analysis, tensile tests, hardness measurements, and fractography revealed significant improvements in mechanical performance, particularly in samples processed with three passes and reinforced with SiO2. While the hardness and strength increased markedly, fracture toughness showed a reduction attributed to porosity and interfacial stress. These findings have potential applications in the development of lightweight structural materials for the automotive and aerospace industries.