In this work, humidity-activated ammonia (NH3) sensor was developed via introducing fumaric acid (FA) into poly(ethylene glycol) diacrylate (PEGDA) by thiol-ene click cross-linking reaction. The iontronic sensor could solve the sensitivity deterioration in high humidity conditions of conventional electronic-based chemoresistive gas sensors. The optimized sensor performs a high response of 24.98–50-ppm NH3 at 70% RH and exhibits good selectivity to NH3 among various interfering gas species. Through the use of quartz crystal microbalance (QCM) measurement and complex impedance spectroscopy (CIS), the humidity-activated NH3 sensing mechanism was thoroughly examined. The as-prepared sensor possesses good stability because of the cross-linked structurally sensitive film. A prototype sensor system was developed to monitor the NH3 concentration at high humidity. The system could sound an alert if the concentration of NH3 exceeds 20 ppm.