We report a dual-pathway decarbonization strategy for high-impact polystyrene (HIPS) that integrates renewable bio-rubbers and marine biomass fillers to reduce reliance on fossil-derived components. Poly(butadiene-co-myrcene) copolymers with 20–50 wt% myrcene were synthesized via neodymium-catalyzed coordination polymerization, achieving high cis-1,4 stereoregularity and molecular weights suitable for impact modification. These bio-rubbers were incorporated in situ during styrene polymerization to produce Bio-HIPS with tunable morphology, transitioning from a salami to a core–shell structure as the myrcene content increased. Concurrently, Caribbean Sargassum biomass was chemically treated to remove non-cellulosic components and used as a 20 wt% bio-filler in both commercial and bio-HIPS matrices. Comprehensive characterization revealed that treated Sargassum enhanced matrix-filler adhesion, improving mechanical properties and maintaining processability. Bio-HIPS composites exhibited increased stiffness, preserved damping capacity, and elevated glass transition temperatures compared to commercial counterparts. This work demonstrates a scalable, sustainable approach to producing high-performance, partially bio-sourced HIPS, valorizing marine waste and advancing circular materials design.