The effect of plant extract of Papaveraceae family Argemone mexicana is studied for use as a low cost and efficient corrosion inhibitor for mild steel in acidic environment. This plant extract is selected for the study of corrosion inhibition in view of its rich source of organic inhibiting molecules as proteins, amino acids, tannins, phenolic compounds, saponins, and flavonoids and nonalkaloids organic compounds such as fused benzene rings, hetero N atom rings, -OCH3, and -OH groups. A simple extraction method is adopted to obtain water-based plant extract. Argemone mexicana extract is for the first time used as an efficient inhibitor for mild steel in 1 M HCl. Weight loss and electrochemical methods are used to study the corrosion. Nearly 80% corrosion inhibition is observed at around 200 mg L-1 concentration and maximum (92.5%) for 500 mg L-1 extract concentration in 1 M HCl. Inhibition mechanism is studied using UV-vis, electrochemical, and surface imaging techniques.