This study focused on improving the dispersibility and phase compatibility of kraft lignin (KL) with polymeric matrices by introducing hexyl alkane groups onto its surface through urethane bonds, yielding hexyl isocyanate-modified KL (KL-HIC) with an 82% degree of substitution (DS). The chemically modified lignin was characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR). This modification enhanced thermoplasticity, decreased the glass transition temperature (Tg) to below room temperature, and introduced a new melting temperature (Tm) at 73.58 °C. Then, KL and KL-HIC were incorporated into a polylactic acid (PLA), a renewable biopolymer, to fabricate biobased films. PLA/KL-HIC films exhibited enhanced hydrophobicity and elongation at break compared with neat PLA and PLA/KL films. Moreover, PLA/KL-HIC films demonstrated significantly enhanced ultraviolet (UV) protection and radical scavenging properties (85.71 ± 0.34%) compared to the neat PLA film (18.35 ± 1.36%). These findings suggest the feasibility of structurally modified lignin employed as a functional additive for performance enhancement and sustainability of agriculture-derived bioplastics.