The environmental concerns about petroleum-based polymers drive the search for sustainable alternatives. This paper investigates sustainable cellulose nanocrystal (CNC) films for packaging applications. CNC films are transparent and provide an excellent barrier against oxygen with moderate performance against water vapor. However, they are brittle and challenging to handle. Incorporating cellulose nanofibrils (CNFs) improves the mechanical properties and handling of the films, enabling the formation of thinner, more manageable films, although transparency decreases. Water vapor permeability remains almost constant, and while oxygen permeability increases, it is still significantly lower than that of most petroleum-derived polymers. The optimal balance of properties was achieved with 25% CNF addition. Structural investigation using micro-CT, scanning electron microscopy, and pycnometry revealed that this optimized composite structure is dense, with less than 0.4% internal porosity, consistent with a structure where CNCs pack around the CNFs, filling the spaces between them.