A green synthesis of Au–Ag bimetallic nanocomposites from aqueous solutions using milk thistle (Silybum marianum) seed extract as a reducing and stabilizing agent has been reported. The formation of the nanocomposites was monitored using UV-vis spectroscopy, and the size distribution and stabilization of the nanocomposites were measured by dynamic light scattering (DLS) and zeta potential. The morphology of the synthesized bimetallic nanocomposite was investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM) with selected area electron diffraction (SAED) analysis. The elemental composition and the surface chemical state of the bimetallic nanocomposite were confirmed by energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis. Fourier transform-infrared (FT-IR) spectroscopy was employed to identify the possible biomolecules responsible for the reduction and stabilization of the nanocomposite. The bimetallic nanocomposite functions as an efficient homogeneous catalyst in the degradation of 4-nitrophenol to 4-aminophenol, in the presence of the reducing agent sodium borohydride. The catalytic reaction kinetics were monitored using UV-vis spectroscopy. The methanol oxidation reaction was performed to study the electro-chemical catalytic activity of the bio-synthesized Au–Ag bimetallic nanocomposite.