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Achieving robust corrosion protection under extreme acidic and high-temperature conditions remains a formidable challenge in the oil and gas industry. In this work, we report a molecularly engineered corrosion inhibitor—a long-carbon-chain-functionalized quaternized pyridine derivative (Q4-AP-C12)—that has exceptional protection for N80 steel in highly aggressive environments at 500 mg/L. The inhibitor achieves corrosion rates of 0.029 g/(m2·h) (99.86% inhibition efficiency) in 1 mol/L (3.6%) HCl solution at 303 K and 7.29 g/(m2·h) (99.21% inhibition efficiency) in 15% HCl solution at 363 K. The superior performance is attributed to a rationally designed synergistic mechanism. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) analyses confirm co-adsorption through electrostatic interaction of the quaternary ammonium group (N+ peak at 402.5 eV) and chemical coordination via the pyridinic nitrogen (N–Fe peak at 399.8 eV), while the dodecyl chain enhances electron-donating ability (elevated HOMO energy at −7.57 eV) and strengthens interfacial adhesion (adsorption energy of −2.62 eV). At the morphological level, all-atom molecular dynamics (MD) simulations reveal that the alkyl chain drives the spontaneous self-assembly of a highly compact and stable bilayer film on the steel surface. This structured film significantly reduces surface roughness to 0.54 μm under aggressive conditions and acts as an effective hydrophobic barrier against corrosive species. By correlating macro-scale performance with atomic-scale interactions, this study provides deep molecular-level insights into the multi-mode adsorption and film formation, offering a robust strategy for designing high-performance inhibitors for demanding oilfield environments.