<p>A key platform in sustainable materials engineering, nanocellulose thin films provide a high-performance, bio-based substitute for synthetic functional layers and polymers derived from petroleum. The structure-property-function correlations of thin films made from three different biomass sources, wood pulp, banana pseudostem, and bacterial cellulose are methodically examined in this review. We assess how important film properties including density, porosity, and fibril orientation are determined by particular processing routes, such as solvent casting, vacuum filtration, spin coating, and freeze-drying. A comparative study shows that bacterial cellulose provides a special 3D connected network with outstanding purity and the highest mechanical integrity for high-performance separators, but wood-based films exhibit high technological maturity and better mechanical strength. Furthermore, we highlight the enablement of waste-derived nanocellulose, especially from banana pseudostems, as a cost-effective “waste-to-value” methodology for antimicrobial and environmental remediation applications, with current limitations in membrane stability and flux under pressure compared to conventional sources. This outlines the current challenges, in particular the high energy consumption of the extraction and the difficulty to transfer laboratory production methods such as spin coating and vacuum filtering to the production line. Finally, we demonstrate that these films can be effectively employed in emerging fields such as flexible electronics, smart packaging, and high selectivity water purification membranes. Furthermore, we provide general strategies which allow to enhance interfacial compatibility and the process efficiency for a real circular bioeconomy.</p>