In this work, three composite membrane materials based on polyaniline (PANI), zinc oxide (ZnO), and graphene were fabricated using chemical oxidative polymerization and integrated with interdigitated electrodes on the flexible printed circuit of an ammonia sensor. This flexible ammonia sensor operates based on changes in membrane resistance with changes in ammonia concentration. The surface morphology and elemental analysis of the sensing composite materials were observed through scanning electron microscopy and energy-dispersive spectroscopy, respectively. The characteristics of the ammonia sensor, such as the thickness effect, response, response time, recovery time, repeatability, and selectivity, were investigated. The planar structure of graphene effectively increased the specific surface ratio and gas reaction surface of the sensor. The fabricated PANI/ZnO/graphene ammonia sensor achieved the highest response of 205% at an ammonia concentration of 20 ppm.