2D transition metal dichalcogenide MoS monolayer quantum dots (MoS-QD) and their doped boron (B@MoS-QD), nitrogen (N@MoS-QD), phosphorus (P@MoS-QD), and silicon (Si@MoS-QD) surfaces have been theoretically investigated using density functional theory (DFT) computation to understand their mechanistic sensing ability, such as conductivity, selectivity, and sensitivity toward NH gas. The results from electronic properties showed that P@MoS-QD had the lowest energy gap, which indicated an increase in electrical conductivity and better adsorption behavior. By carrying out comparative adsorption studies using m062-X, ωB97XD, B3LYP, and PBE0 methods at the 6-311G++(d,p) level of theory, the most negative values were observed from ωB97XD for the P@MoS-QD surface, signifying the preferred chemisorption surface for NH detection. The mechanistic studies provided in this study also indicate that the P@MoS-QD dopant is a promising sensing material for monitoring ammonia gas in the real world. We hope this research work will provide informative knowledge for experimental researchers to realize the potential of MoS dopants, specifically the P@MoS-QD surface, as a promising candidate for sensors to detect gas.