This study investigates kraft lignin, hydrolysis lignin, and tall-oil bio-additives as bio-based extenders for a 70/100 penetration-grade bitumen. The objectives were to evaluate whether kraft lignin behaves predominantly as a reinforcing filler rather than as a more interactive binder modifier, and to assess how tall-oil bio-additives influence rheological and thermal performance. Bio-extended binders were produced using kraft lignin (KLEB−15%) as the core system and benchmarked against hydrolysis lignin (HLEB−15%) and a conventional limestone mastic (LSM−15%) representing an inert filler reference. Composite binders were prepared by adding 5 wt% and 10 wt% of crude tall oil (CTO) or tall oil pitch PN (TOP PN) to KLEB−15%. The chemical composition and thermal stability of unaged binders were determined using Fourier-transform infrared spectroscopy and thermogravimetric analysis, while frequency sweep and multiple stress creep recovery tests characterised the linear and non-linear rheological response. The results indicate that kraft lignin was incorporated predominantly through physical blending and behaved mainly as a reinforcing filler, closely resembling the response of the limestone mastic. In contrast, hydrolysis lignin showed a more modifier-like character with higher elastic recovery and lower non-recoverable compliance. The tall-oil bio-additives acted as effective softening agents: a 5 wt% dosage reduced stiffness while preserving much of the high-temperature performance, whereas 10 wt% CTO caused pronounced softening and compromised rutting resistance. Overall, the findings suggest that kraft lignin’s filler-like behaviour can be modulated by tall-oil additives, while hydrolysis lignin provides a more modifier-type bio-extension. This functional classification offers a viable framework for designing performance-balanced, lignin-extended binders.