Single-walled carbon nanotubes (SWCNTs) hold promise for detecting toxic and chemically aggressive gases due to their unique 1D structure. However, the limited sensitivity of as-produced SWCNT films hampers their practical application as gas sensors. To enhance sensitivity, various functionalization methods, including nitrogen doping, have been explored. In this study, we investigated the impact of low-frequency nitrogen plasma treatment on SWCNT films for sensing reducing and oxidizing air pollutants—specifically, NO2 and NH3—at concentrations ranging from 10 to 50 ppm. TEM and UV-vis-NIR unveiled the etching of the SWCNT structure, and Raman spectroscopy showed the formation of defects following nitrogen plasma treatment. XPS analysis confirmed the presence of amine, pyridinic, and quaternary N-containing defects within the graphitic structure. Upon treating pristine SWCNT films with a 32 W·h dose of plasma, the average chemiresistive response to 50 ppm analyte (in air) at room temperature increased significantly by 100x, rising from 0.2% to 20% (NO2) and from 0.05% to 17% (NH3). Density functional tight-binding modeling corroborated the experimental findings, revealing a correlation between plasma dose and sensor signal. Moreover, the modeling suggested that analyte molecule adsorption on defects is favored over basal plane adsorption.