Abstract: To address the issues of poor degradability and residual environmental pollution associated with the conventional polyethylene (PE) mulch films, a series of fully biodegradable mulch films (PLA/OL/LG) were developed via a synergistic modification of a polylactic acid (PLA) matrix using biomass-derived alkali lignin (LG) and glyceryl tripalmitate (OL). The resulting films were systematically characterized for their microstructures, mechanical properties, thermal behaviors, and compost degradation performance to identify the optimal formula. The results showed that the synergistic effects of appropriate amounts of OL and LG significantly enhanced the toughness of PLA, achieving a favorable balance between stiffness and toughness while maintaining good thermal stability and barrier properties. Compost degradation tests revealed that the modified mulch films exhibited a high biodegradation rate, indicating excellent environmental compatibility. Field application trials further revealed that, compared with the traditional PE mulch films (conventional mulch films), the modified PLA biodegradable mulch films displayed good adaptability to tobacco plant growth and exhibited potential in improving the coordination of chemical constituents and intrinsic quality of tobacco leaves. Sensory evaluation results also confirmed that the application of the modified PLA biodegradable mulch films contributed to the maintenance and enhancement of tobacco leaf sensory quality. Economic assessment indicated that the cost of the modified PLA biodegradable mulch films was comparable to that of traditional PE mulch films, demonstrating promising application feasibility. Therefore, the modified PLA fully biodegradable mulch films provide a viable solution for the application of high-performance and environmentally friendly green mulch materials.