This study investigates the influence of fibrillation of lyocell fibres on the mechanical properties of compression moulded polylactide (PLA), polypropylene (PP) composites, and 3D printed PLA composites. Fibrillation was shown to reduce the strength and elongation at break of the fibres without affecting the Young's modulus compared to untreated fibres. Nevertheless, fibrillation in composites resulted in a 1.15 higher strength for PP composites and 1.62 for PLA composites. Young’s modulus and impact strength were increased by factors of 1.41 and 1.38 for PP composites and 1.2 and 1.23 for PLA composites. Applying the fibrillated fibres in 3D printed PLA shows a significant increase in the mechanical properties. For example, with a fibre mass fraction of 30%, the tensile strength of the composites with fibrillated fibres was increased by a factor of 1.18 compared to composites with untreated fibres. The use of maleic anhydride in the PLA matrix in combination with composite heat treatment further increased the strength by a factor of 1.46. With a strength of 85 MPa, a Young's modulus of 7.2 GPa and an elongation at break of 3.2%, these are some of the highest values reported for this kind of 3D printed materials.