Biomass-derived materials are increasingly being incorporated into plastics to create biocomposites that reduce reliance on fossil-based feedstocks and lower carbon footprints. Maximizing the sustainability potential of these bio-based materials requires increasing their content within polymer matrices. However, a significant challenge arises: as bio-based content increases, performance trade-offs often arise. This study addresses this issue by examining the combined use of multiple bio-based components, specifically lignin and biochar, in acrylonitrilebutadiene-styrene (ABS) biocomposites. The bio-based content reached up to 44 wt%, while retaining adequate processability for extrusion and vacuum forming, as demonstrated by producing a miniature roof box sample. With this biocomposite composition, greenhouse gas emissions could be reduced by up to 40 %. Moreover, the fire performance was slightly improved by adding either lignin or biochar alone, while the combination of both fillers improved the fire performance significantly (a peak heat-release rate being half of that of ABS) due to a synergistic barrier-forming effect, limiting the transport of oxygen and fuel to the heat source and reducing heat transfer. The inclusion of both biochar and lignin influenced the mechanical properties of the composite, leading to an increase (33 %) in stiffness but a slight reduction (22 %) in strength. This study suggests that combining biochar and lignin can maximize bio-based content while improving critical performance characteristics, offering a viable pathway for more sustainable plastics.