•Protection of mild steel in 1M HCl by PCs.•EIS plots showed the adsorptive mechanism of corrosion inhibition.•Polarization results indicate that all studied compounds are mixed-type inhibitors but predominantly cathodic.•The SEM and EDX showed the existence of protective film of PCs on metal surface.•Quantum chemical study provide good support to experimental results. The adsorption and corrosion inhibition effect of three pyridine derivatives namely 2-amino-6-methoxy-4-(4-methoxylphenyl) pyridine-3,5-dicarbonitrile (PC-1), 2-amino-6-methoxy-4-(4-methylphenyl) pyridine-3,5-dicarbonitrile (PC-2) and 2-amino-6-methoxy-4-phenylpyridine-3,5-dicarbonitrile (PC-3) on mild steel in 1M HCl was investigated using gravimetric, electrochemical impedance spectroscopy, potentiodynamic polarization techniques and quantum chemical study. PCs are mixed-type inhibitor mainly cathodic. Activation energy (Ea), equilibrium constant (Kads), free energy of adsorption (ΔG°ads) were calculated and discussed. The studied inhibitors (PCs) obeyed Langmuir's adsorption isotherm. Protective film of PCs on mild steel surface was investigated by using SEM and EDX. Quantum chemical parameters were carried out in the framework of the density functional theory (DFT).