Chemiresistive gas sensors based on semiconducting metal oxides typically rely on noble metal catalysts to enhance their sensitivity and selectivity. However, noble metal catalysts have several drawbacks for practical utilization, including their high cost, their propensity for spontaneous agglomeration, and poisoning effects with certain types of gases. As such, in the interest of commercializing the chemiresistive gas sensor technology, we propose an alternative design for a noble-metal-free sensing material through the case study of Co-doped ceria (Co-CeO₂) catalysts embedded in a SnO₂ matrix. In this investigation, we utilized electrospinning and subsequent calcination to prepare Co-CeO₂ catalyst nanoparticles integrated with SnO₂ nanofibers (NFs) with uniform particle distribution and particle size regulation down to the sub-2 nm regime. The resulting Co-CeO₂@SnO₂ NFs exhibited superior gas sensing characteristics toward isoprene (C₅H₈) gas, a significant biomarker for monitoring the onset of various diseases through breath diagnostics. In particular, we identified that the Co-CeO₂ catalysts, owing to the transition metal doping, facilitated the spillover of chemisorbed oxygen species to the SnO₂ sensing body. This resulting in the sensor having a 27.4-fold higher response toward 5 ppm of C₅H₈ (compared to pristine SnO₂), exceptionally high selectivity, and a low detection limit of 100 ppb. The sensor also exhibited high stability for prolonged response-recovery cycles, attesting to the strong anchoring of Co-CeO₂ catalysts in the SnO₂ matrix. Based on our findings, the transition metal-doped metal oxide catalysts, such as Co-CeO₂, demonstrate strong potential to completely replace noble metal catalysts, thereby advancing the development of the commercially viable chemiresistive gas sensors free from noble metals, capable of detecting target gases at sub-ppm levels.