In the present work, poly(lactic acid) sheets (with a nominal width of 1 mm) reinforced with organically modified montmorillonite (with a mass content of 2.5%) have been manufactured through a reactive extrusion process. A chain extensor (with a mass content of 0.5%) has been used in order to improve poly(lactic acid) properties. The kinetics of the thermal degradation has been analyzed by means of the general analytical equation. Various empirical and theoretical solid-state mechanisms have been tested to elucidate the best kinetic model. In order to reach this goal, master plots have been constructed by means of standardized conversion functions. Given that it is not always easy to visualize the best accordance between experimental and theoretical values of standardized conversion functions, a new index, based on integral mean errors, has been developed to quantitatively discern the best mechanism. By doing that, it has been possible to ascertain that random scission of macromolecular chains was the best kinetic model. The presence of nanoparticles has been beneficial, thus enhancing the thermal resistance of poly(lactic acid).