Dried marjoram leaves (DML) aqueous extract solution was prepared and characterized by Fourier transform infrared spectroscopy (FTIR) and cyclic voltammetry (CV). The effect of DML solution concentration on corrosion protection of mild steel in 0.5M H2SO4 at different temperatures was studied using anodic and cathodic polarization and electrochemical impedance spectroscopy (EIS) measurements. It was determined from the polariazation curves that the DML acts as a mixed type inhibitor since, both the cathodic and anodic reactions of mild steel corrosion were inhibited. The protection efficiency increases with increasing concentration of DML and decreasing temperatures. The surface morphology was investigated before and after adsorption of DML molecules on the steel surface using scanning electron microscope (SEM). The activation and adsorption parameters were calculated and discussed. The results were supported using density functional theory (DFT), and the quantum chemical parameters were estimated. These parameters reveal the effect of the electronic structure of Thymol, the main component of DML, on its electron donating ability at the B3LYP/6-311+G (d, p) level.