Background: Mandibular retrognathism or functional mandibular retrusion is commonly associated with the reduced pharyngeal airway. Functional appliances that are used to treat mandibular retrusion have been found to improve airway dimensions. However, this improvement in the airway has never been assessed in the long-term. Therefore, the purpose of this study was to assess the effects of standard twin-block appliance on pharyngeal airway space in subjects with skeletal class-II pattern and the maintenance of changes achieved after debonding. Materials and methods: Twenty-five subjects (14 females and 11 males) in the age-group of 10–14 (mean 12.08 ± 0.91) underwent treatment of a class II division 1 malocclusion via standard twin-block appliance followed by fixed orthodontic therapy. Pre-treatment (T1), post-twin block (T2), and post-fixed appliance (T3) lateral cephalograms were taken and skeletal and soft tissue landmarks were hand-traced. A pairwise comparison was used to assess whether there were any significant differences in the cephalometric variables of the upper and lower pharyngeal airway between each phase of treatment (T1, T2, T3). Results: There was a significant increase in sagittal dimensions of upper and lower pharyngeal space with a mean increase of 1.34 and 1.26 mm, respectively, following twin-block therapy. There was a significant increase in upper pharyngeal space from pre-treatment till the end of the fixed appliance phase (p < 0.003) and the change in lower pharyngeal space from pre-treatment till the end of the fixed appliance phase was non-significant (p = 0.13). Conclusion: Correction of mandibular retrusion in class II malocclusion subjects by twin-block therapy increased the sagittal dimension of the upper and lower pharyngeal space. This significant improvement in the upper pharyngeal space was maintained till the end of fixed orthodontic treatment. However, the improvement in the lower pharyngeal space was found to be unstable post-debonding.