Tailoring redox-mediators (RMs) and developing systematic fabrication methods for favorable electrochemical kinetics are essential to improve the energy storage performance of fiber-based supercapacitors. The effective use of RMs can provide a unique energy storage mechanism; additional Faradaic redox reactions and optimized ion diffusion between the electrodes and electrolyte can be achieved. Here, we successfully optimized the electrochemical performance of fiber-based supercapacitors using the iodine-based redox mediator (I-RM) potassium iodide (KI). The fiber-based symmetrically yarned supercapacitor cells (f-SYCs), incorporating the KI mediator at a concentration of 7.5 mM, exhibit a high specific capacitance of 13.9 mF at a current density of 10 μA, which directly depicts its superior electrochemical performance compared to that of the previously reported fiber-based supercapacitors. Owing to the limited moisture content present in the polymer-gel electrolyte, the improved electrochemical performance of the f-SYCs containing I-RMs is attributed to the optimized ionic conductivity and diffusion kinetics, as a result of the well-engineered KI electrolyte properties. Synergistically, the results indicate that controlling the amount of RMs in the polymer-gel electrolyte is crucial to achieve excellent overall electrochemical properties in next-generation fiber-based supercapacitors.