We present high performance symmetric quasi-solid-state electrical double layer capacitors (EDLCs) with activated carbon (AC) electrodes produced from peanut-shells. Two different (ethanol soaking and hydrothermal) pre-treatments were given to peanut-shells to tailor ACs' microstructure and comparative supercapacitive performance have been evaluated with Mg-salt (magnesium trifluoromethanesulfonate, Mg (Tf)₂) and ionic liquid, IL (1-ethyl-3-methylimidazolium-trifluoromethanesulfonate, EMITf) incorporated gel polymer electrolytes (GPEs). Morphological and porosity studies indicate larger content of mesoporous interiors in ACs obtained from ethanol pre-soaking, offering superior capacitive performance over hydrothermally-treated ACs. The high room temperature ionic conductivity (∼3.8 × 10⁻³ S cm⁻¹), good electrochemical stability window (∼3.7 V) and flexible nature of the free-standing films of GPEs Mg (Tf)₂/IL/poly (vinylidinefluoride-co-hexafluoropropylene) (PVdF-HFP) show their excellent compatibility with AC-electrodes. Electrochemical impedance spectroscopy and cyclic voltammetry indicate high-rate capability of the device. The AC-electrodes, prepared via ethanol-soaking offer superior performance during charge-discharge tests in terms of specific capacitance (∼189 F g⁻¹), energy (∼26 Wh kg⁻¹) and maximum power (∼57 kW kg⁻¹) with Mg-salt/IL incorporated GPE-film as compared to the devices with only IL-based GPE-film. The EDLC shows stable performance up to ∼10,000 charge-discharge cycles with ∼28% initial fading in specific capacitance. The EDLC is thermally stable in the temperature range from −50 to 70 °C.