Polymer nanocomposite-based sensors were prepared using cellulose acetate (CA), carbon nanoparticles (CNPs), and manganese dioxide (MnO) nanorods to detect and to understand the sensing mechanism of 2-methylcyclopentanone vapor. A sensor with a mass ratio of 1:1.5:3 of MnO/CNPs/CA as well as MnO/CA and MnO/CNP composite and MnO sensors were prepared. The sensor with the three sensing materials combined exhibited an enhancement of response for 2-methylcyclopentanone vapor, ascribed to a synergistic effect between MnO/CNPs/CA. An in situ Fourier-transform infrared (FTIR)-combined online LCR meter setup was used to understand the sensing mechanism of the sensor. The sensing mechanism involved a deep oxidation decomposition of the analyte to CO. This was confirmed from the in situ FTIR-combined online LCR meter results, where a new distinct CO bending mode IR band was recorded. To optimize the performance of the sensor, the composites were prepared by varying the amount of metal oxide added into the composites; sensor A (composition of mass ratio 1:1.5:3), sensor B (composition of mass ratio 2:1.5:3), and sensor C (composition of mass ratio 2.5:1.5:3); their compositions are MnO/CNPs/CA. The performance of sensor B was higher than that of the other two sensors. The sensors also show relatively good response-recovery time. All fabricated sensors were found to have the sensing ability regenerated after the analyte was removed from the system without losing its sensing and recovery abilities. The structural and morphological features of the samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy.