Three quinazolinone compounds, namely 6-chloroquinazolin-4(3H)-one (Q1A), 3-phenethyl-2-thioxo-2,3-dihydropyrido[2,3-d]pyrimidin-4(1H)-one (Q1B), and 6-chloro-3-phenethyl-2-thioxo-2,3-dihydroquinazolin-4(1H)-one (Q1C), have been synthesized, and the inhibition effect of these compounds on the corrosion of mild steel (MS) in 0.5 M H2SO4 has been studied via weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) techniques. These studies revealed that the inhibition efficiency (IE) increased with the concentrations of all the inhibitors studied. The IE of polarization studies is in good agreement with EIS and weight loss measurements and also showed the mixed mode of inhibition by all the quinazolinone compounds. The inhibition was accomplished by adsorption of quinazolinone compounds on the mild steel surface, which was found to obey the Langmuir isotherm. Thermodynamic parameters proved that Q1B and Q1C followed the chemisorption mode, while Q1A followed the physisorption mode of inhibition. SEM studies supported the adsorption of quinazolinone compounds over the metal surface, and the inhibition efficiency follow the order Q1C > Q1B > Q1A.