The present paper reports study on the inhibition of mild steel in 0.5 M HCl with different concentrations of substituted 1,2,4-triazoles using conventional weight loss method, potentiodynamic polarization studies, linear polarization studies, electrochemical impedance spectroscopy and quantum chemical calculations. The theoretical study has been initiated using density functional theory at the B3LYP/6-31G (d) level in order to elucidate the inhibition efficiencies and reactive sites of these inhibitor molecules. The efficiencies of corrosion inhibitors and the global chemical reactivity relate to some parameters, such as E-HOMO, E-LUMO, gap energy (Delta E), electronegativity (chi), global hardness (eta) and the fraction of electrons transferred from inhibitor molecule to the metal atom (Delta N). The calculated results are in good agreement with the experimental data on the whole. In addition, the local reactivity has been analyzed through the Fukui function and condensed softness indices. Results show that the corrosion rate decreases and inhibition efficiency increases with inhibitor concentration. The results of polarization studies reveal that the additive acts as a mixed type inhibitor. The surface adsorption of inhibitor molecules decreases the double layer capacitance and increases the polarization resistance.