Flexible, transparent and biocompatible polymer nanocomposite films can be fabricated using two-dimensional filler materials, which is of great interest for gas barrier applications. A bio-inspired polyvinyl alcohol (PVA) nanocomposite film was fabricated using sulfanilic acid group functionalized MXene (f-MXene) as the reinforcing filler and boric acid (BA) as the crosslinking agent. The f-MXene/BA/PVA nanocomposite films were characterized in terms of their thermal stability, mechanical properties and gas barrier efficacy, and the results indicated these were significantly improved compared to films made from pure PVA and MXene/BA/PVA nanocomposites. This is due to the homogeneous dispersion of the MXene and strong interfacial covalent bonds created by the BA crosslinkers between the f-MXene and the PVA matrix. The nanocomposite film consisting of 0.5 wt% f-MXene and 0.5 wt% BA (0.5 wt% f-MXene/BA/PVA) shows an oxygen permeability of ~0.73 × 10−2 cc∙cm/m2∙day∙atm, which is a reduction of around 69% in gas permeability compared to that of a pure PVA film. In addition, the initial thermal decomposition temperature of the 0.5 wt% f-MXene/BA/PVA nanocomposite film increased from 257.4 °C to 288.2 °C. Futhermore, the tensile strength of the f-MXene/BA/PVA nanocomposite film improved by 67%, and the modulus was also increased by 49%. These results clearly suggest that the MXene/BA/PVA is an effective nanocomposite film when used as a high-performance gas barrier films. The functionalization of MXene and borate-crosslinking represents a practical method to improve the various properties of a range of polymers.