In this study, we tested several amino acids as eco-friendly inhibitors against corrosion of mild steel by used cooking oils (UCOs). The corrosion inhibition was studied by immersing mild steel rods in the UCOs and reference fresh rapeseed and olive oils mixed with amino acids. The immersion tests were conducted at room temperature for three days. The roles of water and bio-oil preservatives (formic and propionic acids) in the corrosion were explored. The mild steel surface morphology changes after exposure to the oils were analyzed with a scanning electron microscope coupled with an energy dispersive spectroscope (SEM-EDS). The concentration of iron dissolved in the oils was determined with a spectrophotometer. A thick layer was analyzed on the surfaces of the mild steel rods immersed in the oils containing formic or propionic acid and water. This layer provided a minor barrier against corrosion. According to the Fourier transform infrared spectrometer (FTIR) analytical results, the layer consisted of an acid and iron salt mixture. All the tested amino acids decreased the concentration of dissolved iron in the UCOs; particularly, cationic amino acids, L-lycine and L-arginine showed adequate corrosion inhibition properties at low concentrations.