Polystyrene clay nanocomposites represent a significant class of tailored materials that possess value-added properties based on their hybrid nature from organic and inorganic chemistry in their framework. Such materials had numerous applications extending from fire retardant properties, gas permeability, and catalysis. Herein, cross-linked polystyrene (CPS) and cross-linked polystyrene- organophilic montmorillonite (OMMT) nanocomposites (VO-MMT-PS) were prepared using 1,4-di(chloromethyl)benzene and ferric chloride as crosslinking agent and catalyst, respectively. The VO-MMT-CPS nanocomposites were prepared with 1, 3, and 5 wt% of MMT with respect to styrene to show the effect of the MMT layers on the properties of the polymer. The CPS and VO-MMT-CPS were post-sulfonated to create strong Brønsted acid sites on the surface of prepared materials. The prepared polymers are described by FTIR, N2–adsorption/desorption isotherms, TGA, and XRD techniques. The low angle XRD showed an increase in the OMMT layer after modification, and exfoliation structures are obtained after polymerization, whereas the entry of the OMMT layer in polymer structure increases the surface area of the polymer. The catalytic effects of the VO-MMT-CPS-SO3H nanocomposites were examined and compared to CPS-SO3H in the methanolysis of oleic acid to form the methyl oleate as a biodiesel model. The VO-MMT-CPS-SO3H that has a high OMMT ratio (5%) represented the highest catalytic performance in the methanolysis reaction with 89% conversion at 70 °C, 4% catalyst, 9:1 methanol: oleic acid molar ratio. Comparing 5% VO-MMT-CPS-SO3H to Amberlyst-15 as a commercial catalyst, the prepared catalysts exhibited better activity and higher thermal stabilities.