This study investigated the corrosion inhibition robustness of an eco-friendly garlic corrosion inhibitor (GCI) for X70 carbon steel in simulated sour oilfield water saturated with hydrogen sulfide (H₂S) environment. The functional groups o GCI were identified using a Fourier-transform infrared spectroscopy (FTIR) technique, confirming the presence of alcohols, phenols, amines, carbonyls, and sulfur-containing compounds, which contribute to adsorption and protective layer formation on the steel surface. The inhibition performance was assessed using the WLM technique, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). At the optimal GCI concentration of 200 mg L⁻¹, the inhibition efficiencies determined by weight-loss, polarization, and EIS measurements were 73.9%, 93.9%, and 96.3%, respectively. The differences among these values are attributed to the distinct principles of the techniques: WLM analysis represents cumulative material loss during immersion, whereas polarization and EIS characterize the electrochemical response of the steel–solution interface. Increasing the inhibitor concentration to 500 mg L⁻¹ produced only marginal improvements in the electrochemical efficiencies, reaching 94.0% by polarization and 96.4% by EIS. More importantly, the potentiodynamic polarization and EIS analyses revealed a mixed-type inhibition mechanism and the formation of a protective film with high polarization resistance. Surface analyses by the SEM and EDX techniques demonstrated the development of a relatively homogeneous organic layer enriched with nitrogen and sulfur, effectively preventing localized corrosion. On the whole, the acquired outcomes demonstrated that GCI is a promising green corrosion inhibitor for corrosion retardation of metals in sour environments, offering an eco-friendly alternative to conventional chemical inhibitors.