The gas-sensing properties of selected transition metal (TM) atoms functionalizing molybdenum disulfide (MoS₂) monolayers as catalysts towards toxic nitrogen-containing gases (e.g., NO and NO₂) were investigated using a combination of density-functional theory (DFT) and non-equilibrium Green's function (NEGF) formalism. Pristine MoS₂ adsorbed NO and NO₂ with relatively weak adsorption energies of −0.11 and −0.19 eV, respectively. To enhance the adsorption mechanism, five doping states were considered, such as (i) sulfur vacancies “V_S” and (ii) Mn, (iii) Fe, (iv) Co, and (v) Ni dopants substituting the S-site in MoS₂. We found that S vacancy-induced and Mn-, Fe-, Co-, and Ni-doped MoS₂ resulted in significantly strong adsorption energies of −2.59 (−2.76), −2.16 (−1.17), −2.87 (−1.85), −3.06 (−1.61), and −1.97 (−0.90) eV for NO (NO₂), respectively. The results of the electronic structure calculations showed that the adsorption of NO and NO₂ drastically changed the magnetic states of the substrate, for instance from paramagnetic to ferromagnetic (FM) semiconducting states (e.g., V_S and Ni-doping) and from FM to either antiferromagnetic (AFM) or paramagnetic semiconducting states (e.g., Mn- or Ni-doping, respectively). The results of current–voltage (I–V) characteristics showed that Co- and Ni-doping yielded the optimal sensor response which was attributed to the changes between two extreme magnetic states, for instance, from FM to paramagnetic semiconducting states and vice versa (e.g., Co- and Ni-doping, respectively). Our refined study of selectivity using seven gases (i.e., CO, CO₂, N₂, O₂, H₂, NO, and NO₂) demonstrated that MoS₂:Co and MoS₂:Ni are potential materials for disposable gas sensors for the capture and the detection of toxic NO and NO₂ gases.