Defects in monolayer MoS2 (M-MoS2) can cause complex electronic states that significantly affect its optical and electrical properties. Understanding and describing the impact of these defects, especially the role of sulfur vacancy (Vs) in M-MoS2 when integrating them into practical technologies, is crucial. However, a significant challenge exists in precisely controlling Vs generation in M-MoS2. This article presents an in situ defect engineering procedure for M-MoS2, considering the influence of external stimuli. We investigated how Vs changes and its impact on the optoelectronic characteristics of M-MoS2 after it is directly exposed to various gas environments. A photodetector device was fabricated, which exhibited an outstanding responsivity of 1.02 × 104 A/W, a detectivity of 1.2 × 1012 Jones, and an ultralow noise equivalent power of 1.56 × 10−18 W Hz−1/2. When the device is exposed to a reducing gas (H2S) environment, the performance increases by 136%, and in an oxidizing gas environment (NO2), it decreases by 68% in terms of responsivity due to a change in the concentration of Vs. We studied the photoresponse characteristics of the device by using Vs as the key parameter. This research contributes to the field of defect engineering in M-MoS2, expanding our knowledge of gas–surface interactions and assisting in producing highly sensitive optoelectronic devices.