An environmentally-friendly method for the synthesis of Au–Ag alloy nanoparticles with controlled composition is proposed. The method involves the simultaneous bioreduction of HAuCl₄ and AgNO₃ using Cacumen Platycladi leaf extract at 90 °C. The formation of the Au–Ag alloy nanoparticles was monitored by recording the absorbance, using UV-visible light spectroscopy as a function of the reaction time and the formation process. The as-synthesized nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy to verify the nature of the alloy. The Fourier transform infrared spectra show that the CC, N–H, (NH) CO, and –OH groups in the C. Platycladi extract served as a reducing agent, whereas the peptides or proteins prevented the aggregation of alloy nanoparticles. The process can be described as a purely “green technique” because no additional synthetic reagents were used as reductants or stabilizers.