A series of poly(N-vinylimidazole)/montmorillonite nanocomposites were prepared by in situ polymerization. This method is the first example of in situ polymerization of VI monomers within the montmorillonite layers for the preparation of nanocomposites at different clay loading degrees. N-vinyl imidazole was first intercalated into the interlayer regions of clay minerals by ion exchange reaction. Then, this organomodified clay was dispersed in the fluid VI monomers at different loading degrees to conduct the in situ polymerization. Polymerization through the interlayer of the clay led to the exfoliated poly(N-vinylimidazole)/montmorillonite nanocomposite formation. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses indicated that the resultant nanocomposites exhibited exfoliated morphologies with homogeneous clay platelet distribution. The PVI/5MMT nanocomposite was selected for the calculation of the decomposition activation energy via the Kissinger method, due to its highest thermal stability. The results showed that activation energy values at both stages for the poly(N-vinylimidazole)/montmorillonite nanocomposite are higher than those of neat poly(N-vinylimidazole), indicating that addition of the clay mineral improves the thermal stability of poly(N-vinylimidazole).