This study explores the use of Allium Cepa Skin Extract (ACSE) as a sustainable, eco-friendly corrosion inhibitor for mild steel, with a focus on industrial applications in acidic environments, such as oil well acidification. The objectives include formulating inhibitors from locally available, cost-effective raw materials, conducting corrosion tests through weight loss methods, and evaluating the influence of immersion time and ACSE concentration on inhibition efficiency. The study also compares ACSE’s performance with the synthetic inhibitor Disodium Hydrogen Phosphate (DSHP) and analyzes ACSE’s phytochemical properties, including alkaloids, tannins, flavonoids, phenolics, and anthocyanins, which are key in corrosion inhibition. Ethanol-extracted ACSE, applied to mild steel samples, achieved a maximum inhibition efficiency of 94.02%, outperforming DSHP’s 39.24%. ACSE’s performance was enhanced by the presence of bioactive compounds such as quercetin and anthocyanins, which form protective layers on metal surfaces. Cost–benefit analysis indicated ACSE’s economic viability over DSHP. The study’s findings demonstrate ACSE’s potential as an environmentally friendly inhibitor that can reduce the industrial reliance on harsh chemicals, with implications for job creation, reduced operational costs for oil companies, and contributions to local content development in line with the Nigerian Content Development and Monitoring Board (NCDMB) mandate. Future research could extend ACSE application to real-world trials, emphasizing collaboration among government, academia, and industry to maximize its potential.