Ammonia is a toxic gas that can cause various respiratory diseases. There are many ammonia sources such as chemical industries, laboratories, and life-stock, in addition to its natural origin. Hence, the detection of ammonia is of utmost importance. Herein, we report a bio-based synthesis of graphene quantum dots using the leaf extracts of Elaeocarpus serratus. The quantum dots were found to emit bright pink colour under a UV lamp. Further, the synthesized quantum dots were complexed with poly(n-vinylpyrrolidone-co-acrylonitrile-co-methacrylic acid) via in situ incorporation. All the synthesized materials were well characterized using various sophisticated techniques. The polymer composite was found to have enhanced electrical properties compared to the original copolymer. At 80 °C, the AC conductivity of the copolymer and the polymer–GQD composite was found to be 1.9 × 10⁻⁷ and 1.6 × 10⁻⁵ S cm⁻¹, respectively. The activation energy of the copolymer was increased from 0.115 to 0.725 on forming the composite. The copolymer showed no ionic nature, whereas the polymer composite was found to be 61.56% ionic in nature. A portable electronic device was fabricated using the polymer composite for the selective and reversible detection of ammonia vapour in the presence of other organic vapours, with a detection limit of 0.232 ppm. A two-fold decrease in the impedance value was observed in the presence of ammonia vapour at room temperature, while the current–voltage characteristic plot showed a five-fold increase in current density at 90 °C in the presence of ammonia vapour. Such a drastic change in the electrical properties of the sensor is attributed to the weak physisorption of the ammonia vapour in the polymer matrix. Furthermore, to check the sensor's practical applications, we studied its impedance response in the presence of the gases released from the rotten fish sample. Interestingly, the sensor showed a significant decrease in the impedance which indicates that the polymer composite could be used for the real-time detection of ammonia.