Gas barrier properties are very relevant in composite materials for applications so diverse such as food packaging, electronics, or old document restoration. In the present work, four different types of cellulose nanofibres (CNFs), two types of clay minerals used individually (sepiolite) or combined (sepiolite + kaolinite), and the influence of pH, were explored in the production of composite films. Neat CNFs, only mechanically treated or prepared by enzymatic pre-treatment, gave films with good mechanical and barrier properties, but the addition of minerals led to a dramatic loss of these properties. Contrarily, the use of thin and functionalized fibrils (TEMPO-oxidised or cationized CNFs) gave composite films with good mechanical, thermal and barrier properties. Superior oxygen barrier properties (oxygen transmission rate (OTR) < 0.4 cm³ m⁻² day⁻¹) were obtained using TEMPO-oxidised CNF and 20% sepiolite, and, in general, for all the composite films containing the TEMPO CNF (OTR ≤ 1.8 cm³ m⁻² day⁻¹). The cationic CNF-based composites also showed a very good oxygen barrier (OTR ≤ 8.2 cm³ m⁻² day⁻¹). The high oxygen barrier could be explained by the compactness of the films and better entanglement of the more fibrillated nanocelluloses with the mineral particles. A decrease in the pH of the suspensions led to a decrease in the film preparation time, without a major negative impact on the composite film’s properties.