A series of nanocomposite materials that consisted of emeraldine base of polyaniline and layered montmorillonite (MMT) clay were prepared by effectively dispersing the inorganic nanolayers of MMT clay in organic polyaniline matrix via in-situ polymerization. Organic aniline monomers were first intercalated into the interlayer regions of organophilic clay hosts and followed by an one-step oxidative polymerization. The as-synthesized polyaniline−clay lamellar nanocomposite materials were characterized by infrared spectroscopy, wide-angle powder X-ray diffraction, and transmission electron microscopy. Polyaniline−clay nanocomposites (PCN) in the form of coatings with low clay loading (e.g., 0.75 wt %) on cold-rolled steel (CRS) were found much superior in corrosion protection over those of conventional polyaniline based on a series of electrochemical measurements of corrosion potential, polarization resistance, and corrosion current in 5 wt % aqueous NaCl electrolyte. The molecular weights of polyaniline extracted from PCN materials and bulk polyaniline were determined by gel permeation chromatography (GPC). Effects of the material composition on the gas barrier property, thermal stability, and mechanical strength of polyaniline along with PCN materials, in the form of both fine powder and free-standing film, were also studied by gas permeability measurements, differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis.