This study investigates the stability and UV-blocking properties of cellulose nanofibril (CNF) and TEMPO-oxidized cellulose nanofibril (TOCNF) films, with and without lignin, under 1000 h of artificial sunlight. The literature to date provides no quantitative analysis of such films’ stability, however such insight is critical for optoelectronic applications for instance solar cells. This contribution examines the films from practical perspectives, considering aging with respect to their optical performance and retention of UV protective qualities. Films containing residual lignin (LignoCNF and LignoTOCNF), and lignin nanoparticles (CNF-LNP and TOCNF-LNP) demonstrated remarkable UV-blocking stability; even after the aging transmittance of LignoCNF and CNF-LNP films remained lower than 1% below 390 nm. Most lignin-containing films exhibited increased transmittance between 400 and 600 nm after aging, except for LignoTOCNF, which showed a decrease in transmittance that was comparable to that displayed by non-lignin films. Nevertheless, long-term light exposure induced a decrease in their mechanical properties. Tensile tests revealed increased brittleness in CNF and LignoCNF, while LNP-containing films showed reduced strain at the break. The observed changes were linked to the potential oxidation of COO- groups and structural modifications in both cellulose and lignin. Overall, the incorporation of lignin into nanocellulose films enhances their durability, UV protection, and mechanical stability, making them promising candidates for sustainable optoelectronic applications.