In this work, six groups of non-toxic compounds were tested as inhibitors for mild steel in 1 M HCl solution: glycine (Gly), glutamic acid (Glu), cysteine (Cys), a mixture of three amino acids glycine, glutamic acid, and cysteine (Gly + Glu + Cys); a mixture of dipeptide composed of glycine and glutamic acid with amino acid cysteine (Gly-Glu + Cys); and a tripeptide composed of glycine, cysteine and glutamic acid which is called glutathione (Glt). The inhibition performances of inhibitor systems for steel corrosion were investigated by electrochemical tests (polarization measurements and electrochemical impedance spectroscopy) and surface analyses (atomic force microscopy-AFM, optical microscope, and photoelectron spectroscopy-XPS). Experimental results showed that all six groups of inhibitors affect the reduction of steel corrosion rate, with Glt having the highest efficiency during 4-h immersion (97.3%). Atomic force microscopy and optical microscope showed that the inhibitors are able to protect the metal surface and reduce the extent of corrosion. The existence of the Glt inhibitory film on the steel surface was confirmed by the XPS method. DFT calculations provided useful insights into adsorption of the corrosion inhibitors.