Inducement of high charge carrier density in organic field-effect transistors (OFETs) is a subject of immense interest due to its fundamental and applied impact on device operation. Here, the high performance of imidazolium-based ionic liquid (IL)-gated OFETs is reported. By exploiting the p-doping of poly(3-hexylthiophene-2,5diyl) (P3HT) in the ambient environment and using the IL-gating, high charge carrier density (1016 cm-2) has been induced in P3HT, which has resulted in high hole mobility (20.2 cm2 V-1 s-1). This is a remarkable improvement in field-effect mobility in organic polymer semiconductors. Along with high mobility, low threshold voltage (∼0.6 V), low subthreshold swing (∼100 mV dec-1), and specifically the high transconductance (>1 mS) position this class of devices as a strong candidate for electrical sensor platforms. The compatibility of ILs with biomolecules and the direct access to IL/P3HT interface make these devices suitable for in situ biosensors. The IL components (cation and anion) were varied (one at a time) to investigate the effect on the electrical performance of the OFETs. An increase in the gate-electrode metal work function leads to a systematic decrease in the threshold voltage (by up to 0.6 V). The mobility exhibits a negative power dependence on specific capacitance, a signature of polaronic self-localization in organic semiconductors. The shelf life of the IL-gated OFETs is found to be more than 40 days. ©