The effects of the reduction process and carbon nanotube (CNT) content on the supercapacitive behavior of electrodes made from flexible, binder-free thick graphene oxide (GO) papers are studied. It is found that the supercapacitive performance depends on several factors, including the presence of oxygenated functional groups after reduction, the interlayer spacing of the GO papers and their wettability with electrolyte. A moderate reduction of GO papers using hydrazine or annealing at a low temperature of 220 °C in air is proven to be more beneficial to achieve a high capacitance than the heavy reduction using a hydrazine vapor or a high temperature thermal treatment. The addition of a small amount of CNT, typically 12.5 wt.%, to form thick GO/CNT sandwich papers gives rise to an excellent specific capacitance of 151 F g−1 at a current density of 0.5 A g−1, as well as a retention ratio of 86% of the initial value after 6000 charge/discharge cycles at 5 A g−1. These improvements arise from the synergistic effects of the increased electronic conductivity and effective surface area associated with large electrochemical active sites due to the presence of intercalated CNT.