This study presents a rheological evaluation of composite binders in which kraft lignin is blended with tall oil bio-additives, crude tall oil (CTO) and tall oil pitch (TOP PN), to mitigate the brittleness induced by lignin. The linear and non-linear viscoelastic properties of these bio-composites were characterised using a Dynamic Shear Rheometer and Multiple Stress Creep and Recovery tests. Results show that lignin substantially increased stiffness and elastic response, enhancing rutting resistance. Both tall oils acted as effective plasticisers, though TOP PN provided a more selective softening that largely preserved this high-temperature performance. This complementary (‘synergistic’) interaction yielded an optimal balance with a formulation of 15% lignin and 5% TOP PN. These findings, based on binder rheology, demonstrate the potential of lignin–tall oil systems for developing sustainable asphalt binders, though further investigation into fundamental interactions and long-term durability is needed to determine practical applicability.