Conductive fibers with enhanced physical properties and functionalities are needed for a diversity of electronic devices. Here, we report very high performance in the thermal and mechanical response of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) microfibers when subjected to an electrical current. These fibers were made by combining the hot-drawing assisted wetspinning process with ethylene glycol doping/de-doping that can work at a current density as high as 1.8 × 104 A cm−2, which is comparable to that of carbon nanotube fibers. Their electrothermal response was investigated using optical sensors and verified to be as fast as 63 °C s−1 and is comparable with that of metallic heating elements (20–50 °C s−1). We investigated the electromechanical actuation resulted from the reversible sorption/desorption of moisture controlled by electro-induced heating. The results revealed an improvement of several orders of magnitudes compared to other linear conductive polymer-based actuators in air. Specifically, the fibers we designed here have a rapid stress generation rate (>40 MPa s−1) and a wide operating frequency range (up to 40 Hz). These fibers have several characteristics including fast response, low-driven voltage, good repeatability, long cycle life and high energy efficiency, favoring their use as heating elements on wearable textiles and as artificial muscles for robotics.