The deterioration of mild steel when exposed to acids, alkalis, and salt solutions is a problem in industrial processes. The corrosion of metals and their alloys has sparked a surge in research efforts to minimize the damage caused by the corrosion process. The research reports the corrosion inhibition properties of Phyllanthus mellerianus stem on mild steel in 1.0 M HCl solution using weight loss and hydrogen evolution techniques. The powdered sample was extracted with ethanol and concentrated with a rotary evaporator. The functional group of the extract, elemental analysis, and morphology of the mild steel were studied with FTIR and SEM. The FTIR analysis confirms the presence of functional groups with nitrogen, oxygen, sulphur, and aromatic rings, whereas the SEM reveals the elements and the morphological structure of the mild steel in the presence and absence of inhibitors in an acid corrodant. Some models were used to extrapolate the inhibition efficiency, enthalpy, enthropy, activation energy, Gibbs free energy, adsorption isotherms, and kinetics investigation. The inhibition efficiency increased with an increase in the concentration of the extract. The values of change in Gibbs free energy obtained at 303K, 313K, and 323K were negative, indicating that the stem extract of Phyllanthus mellerianus was strongly adsorbed on mild steel surfaces and stable at high temperatures. The change in Gibb’s free energy, enthalpy, and activation energy were less than and within the value of 21 kJ/mol. The extrapolation from the thermodynamic and kinetic models shows the effectiveness of the stem extract of PM and confirms the physical (physisorption) adsorption mechanism for the corrosion of mild steel surfaces. The R2 values obtained from the linear regression are strongly fitted to the Langmuir, Temkin, Freundlich, and El-Wadys thermodynic/kinetic isotherms. The inhibitory effectiveness of extracts has been attributed to the presence of the hetero atoms N, O, and S present in the stem extract of Phyllanthus mellerianus.