Thin, high-performance separators are crucial for enhancing the volumetric energy density of electrochemical energy storage devices. Cellulose nanofibril networks are promising candidates due to their intrinsic hydrophilicity and ability to form thin, mechanically strong networks. In this study, wood particles were directly TEMPO-oxidized and fibrillated into nanofibrils (TOCNF) using a microfluidizer, and their resulting networks were tailored with kraft lignin via a simple mixing and casting approach. The influence of lignin content on separator properties and supercapacitor performance was systematically evaluated. The modified networks were compared with pristine TOCNF networks and commercial polyolefin separators. An optimal lignin content of 10 wt% provides the best balance of properties, combining high electrolyte uptake (∼280 wt%), good wet tensile strength (∼6 MPa), and sufficient thermal stability. Supercapacitors assembled with this separator delivered high specific capacitance (112.1 F g−1 at 0.5 A g−1), low series resistance (2.4 Ω), and stable, symmetric charge–discharge behavior. Increasing lignin content to 20 and 30 wt% led to reduced mechanical integrity and electrochemical performance. Overall, the bio-based separators outperformed the commercial reference, highlighting lignin-tailored TOCNF networks as sustainable and high-performance alternatives for next-generation energy storage applications.