The development of high-performance and low-power active components directly integrated onto fibrous substrates is essential for the realization of next-generation wearable e-textiles. Here, we demonstrate high-performance monofilament fiber-based field-effect transistors (FETs) utilizing high-purity semiconducting single-walled carbon nanotubes (s-SWCNTs) and solid-state electrolyte gate dielectrics. To overcome the challenges of patterning on curvilinear fiber surfaces, we developed a fine-thread shadow mask technique, enabling the precise definition of micro-scale channel gaps without complex photolithography. The s-SWcNTs, isolated via a selective conjugated polymer wrapping method, exhibited robust ambipolar transport and high purity, as confirmed by ultraviolet–visible–near infrared (UV–Vis–NIR) and Raman spectroscopy. By integrating a [EMIM][BF₄]/PVDF-HFP ([EMIM][BF₄] = 1-ethyl-3-methylimidazolium tetrafluoroborate, PVDF = polyvinylidene fluoride, and HFP = hexafluoropropylene) solid-state electrolyte, the monofilament fiber-based FETs achieved ultra-low voltage operation (< 1 V) with high areal capacitance (> 1.05 µF·cm⁻²). The resulting fiber transistors exhibited a high hole mobility of 14.7 cm²·V⁻¹·s⁻¹ and a transconductance (g_m/W) of 0.3 S·m⁻¹. This synergistic approach combining textile-compatible fabrication and high-mobility nanomaterials provides a scalable pathway for energy-efficient and in-cloth signal processing and logic circuitry.