In this study, we developed multi-filler poly(methyl methacrylate) (PMMA) composites with enhanced UV shielding and low microwave loss. Integrating SiO2, cellulose nanocrystals (CNCs), and brominated lignin (Br-lignin) effectively tuned interfacial polarization and microstructural distribution, resulting in significantly improved UV blocking performance and stable dielectric behavior at 28 and 40 GHz. The UV blocking efficiency increased from 0.20 for pristine PMMA to 0.85 and 0.82 for the SL2 and K12 systems, corresponding to enhancements of up to 325 and 310%, respectively. These improvements are attributed to the combined effects of structural scattering from SiO2 and charge–transfer interactions from Br-lignin, which promote extended UV attenuation and enhanced light–matter interaction. Among all formulations, the ternary system achieved a balanced multifunctional performance, maintaining strong UV shielding while controlling dielectric loss through stabilized interfacial structures. In contrast, binary systems exhibited limited or sub-additive behavior due to partial interfacial interference. The results indicate that UV shielding and dielectric properties are governed by distinct but interconnected mechanisms, where scattering-dominated processes enhance optical attenuation with minimal dielectric impact, while charge–transfer interactions contribute to both UV absorption and polarization response. The integration of bio-derived fillers enhances circular economy potential and reduces reliance on purely synthetic additives. These findings provide a scalable pathway for designing environmentally responsible polymer composites with optimized optical and electromagnetic performance for advanced sensing applications.