Effective corrosion-protection by plant-extract on metals, in aggressive serviceenvironment, is dependent on the biochemical constituents of which the natural plant is made-up. This paper investigates biochemical characterization of inorganic and organic constituents of Rhizophora mangle L. leaf for gaining insight on its steel-reinforcement corrosion mitigating prospect in NaCl-immersed concretes. Employed for the study include atomic absorption spectroscopy (AAS), Fourier transform infrared (FT-IR) spectroscopy and phytochemical screening techniques. Total-corrosion effect was also studied from steelreinforced concrete samples, having different concentrations of the leaf-extract as admixture, and which were immersed in 3.5% NaCl (simulating saline/marine environment). AAS analyses indicated highest inorganic constituents n Rhizophora mangle L. leaf as iron, Fe = 10,316.17 μg/g, and the least as cadmium, Cd = 6.2019 μg/g, but with neither chromium (Cr) nor lead (Pb). From the leaf-extract, also, FT-IR analyses indicated organic constituents of aromatic chained compounds rich in lone-paired/π-electrons as well as nitrogen, oxygen and sulphur-bearing ligands exhibiting coordinate affinity with iron in reinforcingsteel. Characterization by phytochemical analyses depicted that the leafextract contains tannins, alkaloids, phlobatannins, steroids, saponins, and glycosides. Tested corrosion-inhibition prospects, using the leaf-extract, indicated reduced steel-reinforcement total-corrosion effects that correlated with the extract admixture concentrations employed in the 3.5% NaCl-immersed steel-reinforced concretes