 This work describes optimized conditions for the use of cellulose acetate (CA), a naturally derived polymer, in gel electrolytes (GEs) for electrochromic devices (ECDs). The GEs were doped with lithium perchlorate (LiClO4) as a supporting salt electrolyte and propylene carbonate as a solvent. The conductivity of GEs with varying CA concentration was evaluated by a conductivity meter to determine the effectiveness of electrolyte ion transport in the GE matrix. Then, we employed poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films as an electrochromic layer on an ITO substrate. The P3HT film served as the main cathodic electrochromic layer, the GE as an ion conducting layer, and a piece of bare ITO substrate as an anodic electrochromic layer. We evaluated the stability of the P3HT film by measuring the optical and electrochemical properties of P3HT thin films on ITO in GEs through UV–Vis and cyclic voltammograms obtained during application of potential to the films. Current/voltage data and related performance for ECDs were investigated. The results showed that the prepared GEs have high stability within the operative potential window for ECDs. The electrochromic polymer films operated with the prepared GE showed good stability with fully reversible color change for more than 1000 cycles without polymer film or GE degradation. This work shows the first example of the use of GE with a natural polymer matrix in electrochromic devices and demonstrates their reliability under repetitive switching of applied voltage for up to 1000 cycles.