A PMMA-based polymer electrolyte comprising potassium iodide (KI) and the ionic liquid 1-hexyl-3-methylimidazolium thiocyanate (HMIMI) is described in detail along with its electrochemical, structural, thermal, and photoelectrochemical characteristics. The ionic conductivity studies clearly show that adding HMIMSCN to the PMMA matrix enhances segmental motion and increases ionic mobility by supplying additional charge carriers. The polymer matrix becomes more amorphous upon the addition of KI and HMIMSCN, thereby facilitating ion transport and improving conductivity. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) investigations verified these structural alterations. Additionally, the electrochemical device based on the PMMA-KI-HMIMSCN polymer electrolyte exhibited encouraging performance, suggesting its suitability for energy storage systems such as supercapacitors and EDLCs.