The development of p-channel devices with comparable electrical performances to their n-channel counterparts has been delayed due to the lack of p-type semiconductor materials and device optimization. In this present work, we successfully demonstrated p-channel inorganic thin-film transistors (TFTs) with high hole mobilities similar to the values of n-channel devices. To boost the device performance, the solution-processed copper iodide (CuI) semiconductor was gated by a solid polymer electrolyte. The electrolyte gating could realize electrical double layer (EDL) formation and a three-dimensional carrier transport channel and thus substantially increased charge accumulation in the channel region and realized a high mobility above 90 cm²/(V s) (45.12 ± 22.19 cm²/(V s) on average). In addition, due to the high-capacitance EDL formed by electrolyte gating, the CuI TFTs exhibited a low operation voltage below 0.5 V (V_th = −0.045 V) and a high ON current level of 0.7 mA with an ON/OFF ratio of 1.52 × 10³. We also evaluated the operational stabilities of CuI TFTs and the devices showed 80% retention under electrical/mechanical stress. All the active layers of the transistors were fabricated by solution processes at low temperatures (<100 °C), indicating their potential use for flexible, wearable, and high-performance electronic applications.