Highly efficient thiourea-based organic inhibitors, 1-phenyl-3-pyridin-4-ylmethyl-thiourea (PPMTU), 1-phenyl-4-methyl-3-pyridin-4-ylmethyl-thiourea, and 1-phenethyl-3-pyridin-4-ylmethyl-thiourea were synthesised and used for the first time for the protection of mild steel in 1 M HCl medium. The evaluation of the inhibitors’ effectiveness for protecting mild steel was conducted using the weight loss method, electrochemical impedance spectroscopy, and the Tafel method. The surface characteristics of the metal coupons were characterised using surface characterisation techniques (scanning electron microscopy and atomic force microscopy). The hydrophobic properties of the inhibitors were evaluated by means of contact angle. Langmuir, Freundlich, Temkin, and Frumkin's isothermal models were used to study inhibitor adsorption properties. The best fit was obtained for the Langmuir adsorption isotherm, and the adsorption process is a mixed type rather than physisorption or chemisorption. The inhibitor PPMTU exhibits the maximum inhibition efficiency of 99% by the weight loss method and 98% by the Tafel method at a concentration of just 90 µM. Notably, this is the first work to attain this high inhibition efficiency at a remarkably very low concentration of inhibitor, indicating that a very small amount is sufficient to control the corrosion process by up to 99%. The experimental data are validated using computational methods. The results are promising.