In spite of the excellent thermal properties of nanowires, the typical form factor of unrefined nanowires has restricted their potential use in developing ultralow power electrothermal heater and application device. In this paper, a nanowire array with a perfectly aligned geometric structure on an air-suspended beam is proposed as a novel heating element. By locally confining the generated heat and suppressing beam conduction loss to the substrate, the proposed nanowire array heater overcomes the fundamental power consumption limits of conventional film-type microheaters. As a result, only 2.51 mW of power is required to reach an average temperature of 300 degrees C, surpassing the performance of state-of-the-art microheaters. The developed nanowire array heater is monolithically integrated with gas sensing nanowires to demonstrate its full capability as an ultralow power gas sensor. The fabricated device successfully detects low concentration carbon monoxide gas of 1 ppm, using less than 5 mW of power. The presented technique offers a promising pathway toward realizing always-on gas sensors driven by battery-powered mobile devices, which will ensure a hazardous gas-free safe environment. In addition, the proposed strategy of employing geometrically structured nanomaterials in the electrothermal heater design enables to lead out their potential thermal capability in many other applications.