Polymer structural electrolytes are required to have both superior mechanical properties along with high ionic conductivity in order to develop structural supercapacitors. In the present investigation, a solid polymer electrolyte (SPE) is prepared by varying the relative content of polyethylene glycol (PEG), lithium trfluoromethanesulfonate (LiTf), and graphene nanoplatelets (GnP) contained in an epoxy resin matrix. The mechanical, viscoelastic, and ionic properties are studied as functions of PEG, LiTf, and GnP content. Dynamic mechanical analysis (DMA) is performed to obtain mechanical properties and glass transition temperature, while impedance spectroscopy (IS) is performed to obtain ionic conductivity. It was found that with greater PEG content the ionic conductivity increases, while there is a significant decrease in the mechanical properties. On the other hand greater GnP content results in simultaneously enhancement of both mechanical and ionic properties. Network morphology of these structural electrolytes is studied under scanning electron microscopy (SEM).