The work aims to investigate the effects of nanolignin on the properties and antibacterial activity of chitosan-based films, with the aim of advancing the development and application of functional bio-based films. With phytic acid-modified enzymatically hydrolyzed lignin as the nanoscale functional phase, and carboxymethyl chitosan (CMCh) as the film-forming matrix, the composite films with varying concentrations of nanolignin were prepared via electrostatic interactions. The microstructure, mechanical properties, antioxidant capacity, optical properties, thermal stability, and photothermal antibacterial performance of the films were systematically evaluated. The lignin nanoparticles were uniformly dispersed within the CMCh matrix. As the nanolignin content increased, the tensile strength and elongation at break of the composite films initially increased and then decreased. At a nanolignin content of 10%, the tensile strength increased by approximately 145%, and the elongation at break increased by about 30%. Moreover, the incorporation of nanolignin endowed the composite films with excellent UV resistance, antioxidant capacity, and photothermal antibacterial activity. These nanocomposite films exhibit superior mechanical and photothermal antibacterial properties, providing new insights for the development of functional bio-based plastic packaging materials and antibacterial food packaging.