High-lignin content cellulose nanofibrils (LCNF) were successfully prepared from thermomechanical pulp through TEMPO-catalyzed oxidation, followed by ultrasonic treatment. Preparation protocols of the LCNFs included use of the mild pre-hydrolysis of the thermomechanical pulp and adjustment of sodium hypochlorite loading for the samples with the 23.8 and 14.1 wt.% lignin content, resulting in the increase of the carboxyl group content from 0.70 to 1.24 mmol/g. LCNFs had a diameter of 14 ± 5 nm (AFM evaluation); and the LCNF morphology was affected by contents of both lignin and carboxyl groups. The translucent LCNF films were prepared by solution casting technique. They exhibited the heightened water contact angle of 75-82 • , an increased thermal stability up to 275 • C compared to lignin-free cellulose nanofibril films (39 • and 203 • C, respectively), and excellent UV-blocking ability in a wide spectrum range from 200 to 375 nm. The said LCNFs can be successfully used for manufacturing the packaging materials and/or making the biopolymer composites.