Polymer blend electrolytes composed of poly(vinylidene fluoride-co-hexafluoro-propylene) (PVDF-co-HFP), poly(methyl methacrylate) (PMMA) and 1•0 M magnesium trifluoromethanesulfonate (MgTf3) as salt have been synthesized using solution caste technique by varying the PMMA@PVDF-co-HFP/Mg2+ blend concentration ratio systematically. However, Mg2+ ions interaction with electrode materials and electrolyte molecules results in slow Mg2+ dissociation and diffusion, which in turn leads to inadequate power density and cycle stability. X-ray diffraction (XRD), FTIR, scanning electron microscopy (SEM), Differential scanning calorimetry (DSC), complex impedance spectroscopy, linear sweep voltammetry, AC and DC ionic conductivity studies were used to examine the impact of compositional modification of PMMA in the composite gel polymer electrolyte system. The PVDF-co-HFP/Mg2+ and PMMA@PVDF-co-HFP /Mg2+ mix-based solid polymer electrolyte membrane provides optimal ionic conductivity of 8.014×10-6 and 5.612×10-5 at ambient temperature, and the ionic conductivity of the system rises with increasing PMMA content. Scanning electron microscopy and x-ray diffraction analyses validate the improved ionic conductivity. Electrical conductivity was measured using electrochemical impedance spectroscopy at temperatures ranging from 303 to 363 K. Changes in temperature and PMMA concentration cause an increase in ionic conductivity. Loss tangent and imaginary part of modulus (M"), which relate to dielectric and conductivity relaxation, respectively, demonstrate a quicker relaxation process as PMMA concentration increases up to an optimal value. The modulus (M") demonstrates that the conductivity relaxation is not of the Debye type (broader than the Debye peak).