This work focuses on the corrosion inhibition behavior of a medicinally important and phytochemically richest plant, Boerhaavia diffusa leaves aqueous extract (BDW). The aqueous extract is rich in the quinolone alkaloid 'Lunamarine', whose presence is detected by electronic and IR spectral studies. Theoretical studies on lunamarine using Gaussian 09 and Biovia material studio establishes it as an efficient inhibitor for mild steel corrosion in HCl environment. Electrochemical studies assures that 4(V/V %) BDW has 96% efficiency in 0.5 N HCl, 95% in 1 N and 94% in1.5 N HCl at room temperature. The optimum inhibitor concentration is shown by 4 V/V% BDW in 0.5 N HCl at room temperature. Further increase in inhibitor concentration, acid concentration, temperature etc. decreases the inhibition characteristics of BDW. It is also observed that there is not much difference in the inhibition efficiency with an increase in temperature of the system, suggesting that the mode of adsorption of lunamarine on mild steel surface is predominantly chemisorption. Lunamarine forms a protective monolayer on mild steel surface, which resist the corrosion process, following Langmuir mode of adsorption. Condensed Fukui function analysis of the inhibitor molecule further confirms that chemisorption of the molecule is taking place through O (37) atom of lunamarine. Biovia material studio calculation supports the data generated by Gaussian 09 calculation and Fukui calculation output. XPS elemental analysis on metal specimen before and after exposure in the inhibitor solution affirms the presence of adsorbed lunamarine on the sample surface. Polarization studies assures mixed type behavior for the system in terms of the potential shift. © 2021, The Author(s), under exclusive licence to Springer Nature Switzerland AG.