This study investigated the influence of silver nanoparticles (AgNPs) on the mechanical and antibacterial properties of polycarbonate (PC) and high-density polyethylene (HDPE) composites reinforced with glass (GF), basalt (BF), carbon (CF), and cellulose (CEL) fibres. The mechanical properties of the composites were rigorously assessed by quantifying parameters such as impact strength, tensile strength, and elongation at break. This evaluation was performed in strict adherence to ASTM standards, ensuring reliability and consistency of the obtained data. The results demonstrated that fibre reinforcements enhanced the tensile strength of the composites, but resulted in reductions in both impact strength and elongation at break, particularly in composites containing cellulose. The addition of AgNPs further decreased impact strength and elongation at break, while slightly reducing tensile strength. The antimicrobial properties were evaluated using bacterial strains Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Composites reinforced with glass fibre and AgNPs exhibited the highest antimicrobial efficacy compared to those containing AgNPs combined with cellulose, carbon, or basalt fibres. These findings suggest that while AgNPs enhance the antimicrobial properties, they may compromise the mechanical integrity of fibre-reinforced composites. The study contributes to the development of advanced composites with multifunctional properties for medical, sports, aerospace, construction, and engineering applications.