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A series of acridines were designed and synthesized for the development of effective inhibitors for mild steel corrosion in 1 M HCl solution, in which the halogen-substituted acridines showed better inhibitive performance than the nonhalogen-substituted acridines. The corrosion protection properties of the halogen-substituted acridines, including 2-chloro-9-phenylacridine (CPA), 2-chloro-9-(2-fluorophenyl)­acridine (CFPA), and 2-bromo-9-(2-fluorophenyl)­acridine (BFPA), were further investigated using weight loss test and electrochemical techniques. The results indicated the halogen-substituted acridines have excellent inhibitiion performance, and these acridines act as mixed-type inhibitors with predominant cathodic effectiveness. Adsorption of acridines on a mild steel surface obeyed the Langmuir adsorption isotherm. The adsorption of the inhibitor molecules on the steel surface was further supported by scanning electron microscopy (SEM), scanning electrochemical microscopy (SECM), and FTIR spectroscopy. The inhibition mechanism of the investigated halogen-substituted acridines was derived using DFT-based quantum chemical calculations for their neutral as well as protonated forms. Both experimental and DFT studies suggested that the inhibition efficiency of three halogen-substituted acridines followed the order of η­(BFPA) > η­(CFPA) > η­(CPA).