Carbon based conductive polymer composites as electrothermal materials offers many merits in high flexibility, light weight, low cost, excellent processability, and rapid heating ability, however, the maximum working temperature is usually lower than 200 °C, hampering seriously the medium–high temperature applications. Herein, a series of rationally designed glass fiber reinforced graphite/carbon black@polyethersulfone (G/CB@PES) composites are developed to significantly promote the maximum working temperature over 300 °C. To be specific, the introduced glass fibers acted as rigid skeleton greatly enhance the mechanical strength of PES polymer matrix, thereby offering robust structural stability in high temperature working scenario. Meanwhile, glass fibers not only endow G/CB@PES composite with excellent flame-retardant ability, but also provide rapid thermal conduction channels to reduce the risk of heat accumulation and overheating. More importantly, glass fiber reinforced G/CB@PES composites as electrothermal film heaters exhibit superior electric heating performance in terms of fast temperature responsiveness, high electric heating efficiency, stable maximum working temperatures, and high electric power efficiency under the relatively low applied voltages of 3–21 V. This work provides new insights on the innovative design and facile fabrication of high working temperature electrothermal film heaters in emerging applications.