Monolayer (ML) MoS2 has become a very promising two-dimensional material for photorelated applications, potentially serving as the basis for an ultrathin photodetector, switching device, or transistors because of its strong interaction with light in ambient conditions. Establishing the impact of individual ambient gas components on the optical properties of MoS2 is a necessary step toward application development. By using in situ Raman microspectroscopy with an environment-controlled reaction cell, the photoluminescence (PL) intensity of chemical vapor deposition (CVD)-grown MoS2 MLs is monitored at different intralayer locations under ambient and controlled gas environments, such as N2, O2, and H2O. This new approach enables us to monitor the optical properties of MoS2 at different locations on the flakes and separate the role of photoreaction of various gases during laser irradiation. Upon mild photoirradiation in ambient conditions, the PL intensity in the interior of the ML MoS2 flakes remains unchanged, while the PL intensity at the edge region increases drastically. Photoirradiation in controlled gas environments reveals that O2 is necessary to increase the PL intensity at the MoS2 flake edges, attributed to the charge transfer of chemisorbed O2. N2 or H2O and N2 environments induce decreasing PL intensity upon repetitive laser irradiation. However, the H2O and O2 gas mixture, a combination designed to mimic ambient conditions, is necessary to maintain the PL intensity at the interior of the ML MoS2 flakes. Our study demonstrates that photoreactions with the gaseous environment on the MoS2 ML flakes should be taken into consideration even upon mild photoirradiation because they strongly impact the flakes’ optical properties.