Cysteine based silver-gold nanocomposite (Cys/Ag-Au NCz) was synthesized, this was followed by its characterization using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Ultraviolet-visible spectroscopy (UV-Vis), Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDAX), Thermogravimetric analysis (TGA) and Transmission electron microscopy (TEM). Cys/Ag-Au NCz was studied as novel green corrosion inhibitor for mild steel in 1M HCl solution at varying concentration and temperature using gravimetric, Potentiodynamic polarization (PDP), Electrochemical impedance spectroscopy (EIS), SEM, EDAX and FTIR. Weight loss, PDP and EIS studies confirmed Cys/Ag-Au NCz as efficient corrosion inhibitor at moderately low concentration. The maximum inhibition efficiency of 96 % was observed at 303 K at 300 ppm. Cys/Ag-Au NCz acted by affecting both anodic and cathodic processes and its adsorption on steel surface followed the Langmuir adsorption isotherm. EIS data displayed the existence of protective film at mild steel/solution interface in Cys/Ag-Au NCz inhibited system. SEM micrograph in presence of Cys/Ag-Au NCz inhibited acid solution displayed better morphology as compared to blank solution. The UV-Vis and FTIR data indicates good interaction between the Cys/Ag-Au NCz and steel surface.