A cellulose acetate phthalate (CAP)-mannitol copolymer dispersed in the polyvinly alcohol (PVA) film was in situ mixed with the copper (Cu) metal nanoparticle (NP)-dispersed carbon nanofibers (CNFs). The synthesized polymer-metal-carbon nanocomposite was efficiently used as the chemiresistive sensor material for determining the quality (freshness) of banana via the measurement of electrical current during the release of ethylene (C2H4) gas from the banana samples. The prepared material was characterized for its physico-chemical properties, using different spectroscopic techniques such as scanning electron microscopy, atomic force microscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy. The detection and quantitative limits for C2H4 gas were determined to be similar to 45 and 136 parts per billion, respectively, using differential pulse voltammetry measurements. Amperometry measurements showed a reversible response of the sensor within 30 s of the exposure of C2H4 gas. The efficient performance of the proposed material sensor was attributed to the formation of a Cu-C2H4O complex by the catalytic graphitic CNF surface and Cu metal nanoparticle, COOH functional groups containing CAP having entrapping ability for C2H4, and increased electrical conductivity of the CAP-mannitol copolymer. The method for the synthesis of the sensor material was simple, and the response of the fabricated sensor towards ethylene detection was fast. The chemiresistive sensor developed in this study may be used for determining the quality of fruits that release C2H4 during their various developmental stages.