Pitch-based carbon fibers with multi-walled carbon nanotubes (MWCNTs) were fabricated via an electrospinning method and used as gas sensor electrodes. The pitch-based carbon fibers were treated at various temperatures to investigate the effect of the reaction temperature. The electrospun fibers were thermally treated to produce carbon fibers, and the resulting material was chemically activated to increase the number of active sites for efficient gas adsorption. The activation process improved the porous structure by increasing the specific surface area by approximately 86-fold. Due to the improved porosity and electrical conductivity, gas adsorption sites were enlarged and electron transfer was improved, resulting in a high-performance NO gas sensor with improved sensitivity and rapid response time. The improved porosity was attributed to the chemical activation process, and the enhanced electrical conductivity was attributed to the heat treatment and the addition of MWCNTs.