This work offers a detailed examination of the corrosion inhibiting action of an aqueous extract derived from Eichhornia crassipes (water hyacinth), employed in conjunction with zinc oxide (ZnO) nanoparticles, as a hybrid green formulation for mild steel exposed to 1.0 M HCl. The study confronts two interrelated objectives: reducing metallic deterioration in highly acidic media and repurposing an invasive aquatic weed into a usable material resource. A series of electrochemical measurements, including impedance spectroscopy and potentiodynamic polarization, were carried out over a range of extract to nanoparticle ratios. The optimal composition 0.5 g of plant extract combined with 0.1 g of ZnO yielded a maximum efficiency of 55.73%, a value substantially higher than that recorded for either constituent alone, thus confirming a cooperative enhancement effect. Polarization data further revealed that the combined system acts as a mixed inhibitor, moderating both anodic metal dissolution and cathodic hydrogen discharge. Microscopic and spectroscopic surface analyses (SEM–EDX) provided clear evidence of a dense and well adhered film covering the steel substrate, which curtailed both localized pitting and widespread surface loss. The presence of naturally occurring phytochemicals tentatively identified as flavonoids, tannins, and phenolics together with the inherent surface activity of ZnO nanoparticles, appears to facilitate improved adsorption and prolonged film resilience. This chemical interplay forms the basis for the observed rise in protective performance. Beyond the laboratory findings, the study underscores the practical viability of transforming an ecological nuisance into an inexpensive, non-hazardous inhibitor, positioning it as a plausible substitute for conventional but hazardous compounds such as chromates and synthetic organic inhibitors. In a broader context, the results support the principles of waste valorization and resource cycling, while pointing toward future work on nanoparticle surface modification and improved extraction protocols.