We have demonstrated a highly sensitive novel platform for real-time detection of Mercury (Hg2+) ions after successfully making silver nanowires (AgNWs)-MoS2 nanocomposite and functionalizing it over ungated AlGaN/GaN high electron mobility transistor (HEMT). The AlGaN/GaN HEMT structures were grown over the sapphire substrate using Molecular Beam Epitaxy. AgNWs-MoS2 nanocomposites were optimized for the device functionalization and 1:4 ratio was found highly sensitive for Hg2+ ions. The sensor exhibits high sensitivity towards Hg2+ ions of 1.604 mA/ppb and calculated its Limit of Detection (LoD) up to the range of 20 ppt. The observed sensitivity is highest among previously reported AlGaN/GaN fabricated HEMT based sensors for Mercury (Hg2+) ions detection and is well below the standard permissible limits as set by World Health Organization (WHO) and Environmental Protection Agency (EPA). The enhancement in sensitivity is due to the enhanced surface to volume ratio of AgNW-MoS2 nanocomposite and the highly conductive nature of silver nanowires incorporated in MoS2. Moreover, we also performed sensing on real water samples of tap water and lake water. Further, we showed the smart sensing capability of our developed sensor by illustrating the Internet of Things (IoT) enabled system for next-generation heavy metal ion sensing. IEEE