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The influence of new quinoxaline derivative namely (E)-3-(4-methylstyryl) quinoxalin-2 (1H)-one (QNMS) on the corrosion of mild steel in 1 M HCl was studied using using Tafel polarization and electrochemical impedance spectroscopy. Quantum chemical calculations were performed using methods based on density functional theory (DFT/B3LYP) and Monte Carlo simulations (MCs). The inhibitor studied [(E)-3-(4-methylstyryl) quinoxalin-2 (1H)-one (QNMS)] showed a maximum inhibition efficiency of 91% at 10–3 M. Adsorption of QNMS on mild steel surface follows the Langmuir adsorption isotherm in order to discover the mode of adsorption process, various thermodynamic and activation parameters were evaluated. Potentiodynamic polarization studies show that QNMS compound acts as a mixed inhibitor. Data obtained from EIS measurements were analyzed to model the corrosion inhibition process through the appropriate equivalent circuit model. The EIS data bring that, the development of a protective layer of QNMS increases the charge transfer resistance and decreases the double layer capacitance of mild steel in acid medium. Quantum chemical calculations were employed to study the electronic properties of QNMS to ascertain the correlation between the inhibitory effect and the molecular structure. Both the experimental and theoretical (EHOMO, ELUMO, ΔE, µ, ΔN,... and Monte Carlo simulation) results are in good agreement with each other in this regard and confirm that QNMS is an effective inhibitor.