A LiPON–WO3 composite thin film (LPWON) was evaluated for use as a solid electrolyte in solid state electrochromic (EC) devices. LiPO4 and a WO3 (2wt%) composite sputtering target was synthesized by a ball milling process. The LPWON thin films were deposited by RF magnetron sputtering in Ar+N2 and N2 atmospheres. The structural, electrochemical, and optical properties of the LPWON electrolytes were characterized by X-ray diffraction (XRD), UV–visible spectroscopy, and an impedance analyzer. EC mirrors with WO3 (coloring layer), LPWON (solid electrolyte), and stainless steel (mirror electrode) on ITO (transparent electrode) glass were fabricated to analyze the improved EC properties due to the LPWON electrolyte. The LPWON may lead to electrolytes with more stable potential cycle properties.