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 "VS" 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., VS 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.