Graphene has attracted extraordinary attention for gas sensing due to its large specific surface area as well as its high charge carrier mobility. Nonetheless, in most cases, graphene derivatives, such as reduced graphene oxide (rGO), were employed as sensing elements instead of pristine graphene. In this contribution, pristine graphene noncovalently functionalized by a biocompatible stabilizer (flavin monocleotide sodium salt, FMNS) was produced for the application as NH3 sensing materials in a chemiresistor type gas sensor. Detailed characterizations indicate that the graphene flakes exhibit good structural quality with few defects. The optimized ammonia sensors demonstrate outstanding performance: ultralow limit-of-detection (1.6 ppm), excellent sensitivity (2.8%, 10 ppm; 18.5%, 1000 ppm), reproducibility, reversibility, low power consumption (work temperature, 25 °C) as well as low cost. Additionally, the roles of FMNS from graphene preparation to NH3 sensing are elucidated via all-atom molecular dynamics simulations: (1) stabilizer for the graphene dispersion, (2) p-type dopant for graphene-based sensing element, and (3) active adsorption sites for NH3 gas sensing. This contribution provides an efficient strategy to design highly sensitive pristine graphene-based NH3 gas sensors utilizing FMNS-like molecules, involving a facile and environmentally friendly process, biocompatible materials, low cost equipment, and scale-up capability.