The utilization of natural fibers and fillers in composite materials has gained significant attention for their potential to enhance mechanical, thermal, and tribological properties while promoting sustainability. In this study, Prosopis juliflora fiber (PJF) reinforced epoxy composites, treated with silane and incorporating Syzygium cumini filler (SCF), were investigated to assess their performance across various parameters. The composites were subjected to comprehensive mechanical, tribological, thermo-gravimetric, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), 3D profilometer, and morphology studies to elucidate the effects of silane treatment and SCF content on composite properties. The results of the mechanical characterization revealed significant improvements in tensile strength, impact strength, and microhardness in silane-treated composites with higher SCF content compared to untreated samples and those with lower SCF content. Specifically, the ST/SCF3 composite exhibited the highest values, with a tensile strength of 118.65 ± 4.63 MPa, impact strength of 27.83 ± 1.96 kJ/m2, and microhardness of 91.63 ± 2.84 HV. Tribological analysis indicated enhanced wear resistance and reduced coefficient of friction in silane-treated composites with higher SCF content, with the ST/SCF3 composite demonstrating the lowest wear loss (62 μm) and frictional force (5.2 N). Thermal characterization revealed improved thermal stability in silane-treated composites, particularly in the ST/SCF1 and ST/SCF3 formulations, suggesting increased resistance to temperature-induced degradation.