A thermogravimetric study in both nitrogen and air atmospheres has been carried out on unfilled and graphene-reinforced solid and foamed polycarbonate. Polycarbonate foams were prepared using a supercritical CO2 dissolution one-step batch foaming process. Results showed that polycarbonate displayed a characteristic one-step decomposition under nitrogen, while three-step degradation was observed in air. In addition, as-received pristine graphene nanoplatelets displayed a three-step degradation in air, compared to a mild degradation under nitrogen. It was found that the thermal stability remarkably improved for the foamed composites, related to a combination of a heat transfer reduction promoted by the insulating cellular structure and the presence of the platelet-like graphene, which helped create a physical barrier effect, delaying the escape of volatile products generated during decomposition.