A new corrosion inhibitor namely poly(vinyl alcohol-leucine) composite (PVAL) has been synthesized and its influence on corrosion inhibition of mild steel in 1 M hydrochloric acid solution has been studied by weight loss and potentiodynamic polarization techniques. The composite (PVAL) showed more than 95% inhibition efficiency (IE) at an optimum concentration of 0.6% by weight. The inhibition efficiency of inhibitor has been found to vary with inhibitor concentration, solution temperature, and immersion time. Various kinetic and thermodynamic parameters (Ea, ΔHo, ΔSo for corrosion and ΔGads, ΔHads, ΔSads for adsorption) reveal a strong interaction between inhibitor and mild steel surface. The negative values of ΔGads indicate the spontaneous adsorption of the inhibitor on mild steel surface. Potentiodynamic polarization studies showed PVAL as mixed type inhibitor. It inhibits mild steel corrosion by blocking the active sites of the metal. Electrochemical impedance spectroscopic (EIS) techniques were also used to investigate the mechanism of corrosion inhibition.