Defect engineering of 2D transition metal dichalcogenides (TMDCs) is essential to modulate their optoelectrical functionalities, but there are only a few reports on defect‐engineered TMDC device arrays. Herein, the atomic vacancy control and elemental substitution in a chemical vapor deposition (CVD)‐grown molybdenum disulfide (MoS₂) monolayer via mild photon irradiation under controlled atmospheres are reported. Raman spectroscopy, photoluminescence, X‐ray, and ultraviolet photoelectron spectroscopy comprehensively demonstrate that the well‐controlled photoactivation delicately modulates the sulfur‐to‐molybdenum ratio as well as the work function of a MoS₂ monolayer. Furthermore, the atomic‐resolution scanning transmission electron microscopy directly confirms that small portions (2–4 at% corresponding to the defect density of 4.6 × 10¹² to 9.2 × 10¹³ cm⁻²) of sulfur vacancies and oxygen substituents are generated in the MoS₂ while the overall atomic‐scale structural integrity is well preserved. Electronic and optoelectronic device arrays are also realized using the defect‐engineered CVD‐grown MoS₂, and it is further confirmed that the well‐defined sulfur vacancies and oxygen substituents effectively give rise to the selective n‐ and p‐doping in the MoS₂, respectively, without the trade‐off in device performance. In particular, low‐percentage oxygen‐doped MoS₂ devices show outstanding optoelectrical performance, achieving a detectivity of ≈10¹³ Jones and rise/decay times of 0.62 and 2.94 s, respectively.