Simple and easily integrated design of flexible and transparent electrode materials affixed to polymer based substrates hold great promise to have a revolutionary impact on the functionality and performance of energy storage devices for many future consumer electronics. Here, we demonstrate an environmentally friendly, simple yet scalable approach to produce optically transparent and mechanically flexible all-solid-state supercapacitors. These supercapacitors were constructed on tin-doped indium oxide (ITO) coated polyethylene terephthalate (PET) substrates by intercalation of a polymer-based gel electrolyte between two reduced graphene oxide (rGO) thin film electrodes. The rGO electrodes were fabricated simply by drop-casting of graphene oxide (GO) films, followed by a novel low-temperature (~250 oC) vacuum-assisted annealing approach for the in-situ reduction of GO to rGO. A trade-off between the optical transparency and electrochemical performance is determined by the concentration of the GO in the initial dispersion; whereby the highest area specific capacitance (~ 650 μF cm-2) occurs at a relatively lower optical transmittance (24%). Additional experiments demonstrated that the devices are mechanically flexible and exhibit stable cycling with a capacity retention rate above 90% under various bending angles and cycles.