Spin-polarized density-functional theory (DFT) has been employed to study the effects of atmospheric gases on the electronic and magnetic properties of a defective transition-metal dichalcogenide (TMD) monolayer, MoX₂ with X = S or Se. This study focuses on three single vacancies: (i) molybdenum “V_Mo”; (ii) chalcogenide “V_X”; and (iii) di-chalcogenide “V_X2”. Five different samples of sizes ranging from 4 × 4 to 8 × 8 primitive cells (PCs) were considered in order to assess the effect of vacancy–vacancy interaction. The results showed that all defected samples were paramagnetic semiconductors, except in the case of V_Mo in MoSe₂, which yielded a magnetic moment of 3.99 μ_B that was independent of the sample size. Moreover, the samples of MoSe₂ with V_Mo and sizes of 4 × 4 and 5 × 5 PCs exhibited half-metallicity, where the spin-up state becomes conductive and is predominantly composed of $d_{xy}$ and $d_{{\mathrm{z}}^2}$ orbital mixing attributed to Mo atoms located in the neighborhood of V_Mo. The requirement for the establishment of half-metallicity is confirmed to be the provision of ferromagnetic-coupling (FMC) interactions between localized magnetic moments (such as V_Mo). The critical distance for the existence of FMC is estimated to be $d_c\cong$ 16 Å, which allows small sample sizes in MoSe₂ to exhibit half-metallicity while the FMC represents the ground state. The adsorption of atmospheric gases (H₂O, O₂, O₃) can drastically change the electronic and magnetic properties, for instance, it can demolish the half-metallicity characteristics. Hence, the maintenance of half-metallicity requires keeping the samples isolated from the atmosphere. We benchmarked our theoretical results with the available data in the literature throughout our study. The conditions that govern the appearance/disappearance of half-metallicity are of great relevance for spintronic device applications.