Corrosion behaviour of mild steel in 1M Phosphoric acid with Bacampcillin as corrosion inhibitor has been studied by using weight loss, Potentiodynamic polarization, Electrochemical impedance spectroscopy, Hydrogen permeation and diffuse reflectance spectroscopic studies. All these techniques reveal that inhibition efficiency increases with the increase in the concentration of inhibitor. Polarization studies indicated that inhibitor behaved as cathodic inhibitor. Diffuse reflectance spectroscopy confirmed the adsorption of inhibitor on the mild steel surface obeying Langmuir adsorption isotherm. A Gaussian simulation technique was used to track the quantum mechanical analysis and recognized correlations between different types of descriptors and measured corrosion inhibition efficiency for inhibitor. The quantum chemical analysis demonstrates the inhibition efficiencies of the compound determined by electrochemical methods.