High mechanical performance applications often rely on fibre-reinforced composites using continuous synthetic fibres and petrochemical-based polymers. A shift towards bio-derived fibres and bio-derived polymers can reduce the reliance on non-sustainable materials. Long/continuous high-strength bio-derived fibres, such as flax and viscose, offer the potential to achieve mechanical properties similar to synthetic fibre composites. Polylactic acid (PLA), a sustainable bio-derived thermoplastic polymer known for its relatively high strength and stiffness compared to other polymers, is a promising alternative to petrochemical-derived counterparts. This study used long flax fibres and continuous viscose fibres in yarn form to reinforce PLA matrices, resulting in high-performance composite filaments. These filaments are designed for a recently developed and evolving manufacturing method, Fused Deposition Modelling (FDM) 3D printing, for which product performance has been limited by the commercially available filaments. Achieving high-performance FDM filaments requires addressing challenges related to fibre wetting and uniform fibre/polymer distribution, which this study addresses through impregnation and consolidation techniques. The research covers comprehensive experimental investigations into the physical, mechanical, and thermal properties of viscose, flax fibres, and PLA matrices. Successful outcomes include the development of long/continuous bio-derived fibre-reinforced composite filaments by employing two impregnation methods: solution and emulsion impregnation. Analysis covers the morphology, porosity, fibre weight fraction, and mechanical and thermal properties of the composite filaments and 3D printed composites. Composite filaments were produced using three reinforcement yarns in their as-received form: viscose, two types of flax – standard flax and bleached flax. Viscose fibres were purchased in the form of fully continuous bio-derived fibres, while flax fibres were long and discontinuous, twisted into continuous yarns. Two PLA grades, PLA 2003D (in the form of granules) and PL 1005 (in the form of a PLA/water emulsion), were employed for solution and emulsion impregnation, respectively. Tensile testing of single fibres and matrices yielded results consistent with literature values. Bleached flax fibres exhibited the highest tensile strength (921.6 MPa), standard flax fibres displayed the highest Young’s modulus (31.8 GPa), and viscose fibres demonstrated the highest elongation at break (13.2%). PLA 2003D exhibited the highest tensile properties of the two PLA grades. Thermal stability, as assessed by TGA, demonstrated stability of both bio-derived reinforcements and matrices up to 250 oC. DSC analysis confirmed the semi-crystalline nature of PLA 2003D and the amorphous nature of PL 1005. An impregnation and consolidation methodology was developed to produce composite filaments using both PLA solution and emulsion. Initially employing a single impregnation bath, it was later upgraded to dual baths in tandem for enhanced efficiency. A yarn spooler directed the fibre into the impregnation bath with squeezing rollers, followed by excess resin removal through a nozzle. The impregnated filament was collected on a winding mandrel and passed through a heated consolidation die. Various formulations with different impregnation cycles were tested for solution and emulsion impregnation methods. Polymer uptake depended on the affinity between the fibre and impregnating liquid, while yarn twist influenced polymer distribution. Emulsion impregnated composites exhibited higher polymer uptake due to better affinity between water and bio-derived fibres. However, solution impregnated composites showed better polymer distribution, particularly for twisted flax-reinforced filaments, relating to the efficient impregnation of PLA/DCM solution. For both impregnation methods, standard flax and bleached flax reinforced composites exhibited improved interfacial adhesion compared to viscose, attributed to mechanical interlocking between the rough surface of flax fibres and PLA in contrast to smooth viscose fibres. For PLA/viscose composites, emulsion impregnation resulted in higher tensile strength (254.7 MPa) and Young’s modulus (9.1 GPa), surpassing reported values in the literature. On the other hand, for PLA/standard flax and PLA/bleached flax, solution impregnation yielded superior tensile properties. Solution impregnated PLA/bleached flax, for the 7wt% x 3 formulation, had the highest tensile strength (356.1 MPa), while the 7wt% x2 tandem formulation showed the highest Young’s modulus (17.6 GPa) of all composites. The achieved tensile strength for PLA/bleached flax in this study also exceeded values reported in existing literature for PLA/flax composites. The highest-performing formulations for each reinforcement, employing both solution and emulsion impregnation, were chosen to produce 3D printing filaments. A Makergear® FDM 3D printer, compatible with filament diameters of 1.4 to 2 mm, was utilized. Three filaments were pultruded together into one using a consolidation die to achieve the required filament diameter. PLA/viscose and PLA/bleached flax were selected for 3D printing in both solution and emulsion impregnation formulations, following an analysis of morphology, porosity, fibre weight fraction, and tensile properties of the multiple consolidated filaments. To further optimize diameter and enhance printability, the multiple consolidated filaments underwent melt-impregnation with additional polymer, resulting in 1.45±0.5 mm diameter filaments. Characterization of the 3D printing filaments revealed a reduction in tensile strength and Young's modulus due to increased polymer content. Optical microscopy and porosity analysis of the 3D printed composites revealed increased porosity in the printed specimens compared to the filaments due to voids between the printed layers and pulling of the fibres onto the surface of the printed part because of the nozzle's drag force exerted during the printing process. Emulsion impregnated PLA/viscose exhibited lower porosity and printing defects compared to other composites relating to better affinity between PLA emulsion and viscose as mentioned earlier. Tensile strength and Young’s modulus values of the 3D printed composites decreased compared to the filaments due to printing induced defects mentioned previously. The highest tensile and flexural properties were obtained for emulsion impregnated PLA/viscose composites, demonstrating the significance of efficient impregnation. The 3D printed emulsion impregnated PLA/viscose composites displayed an impact strength greater than 127KJ/m2, which is higher than what has been reported in the literature for any PLA biocomposite.