Fused Filament Fabrication is an Additive Manufacturing method that can produce bespoke components due to the high number of printing parameters available. Each of them plays a significant role in the final mechanical properties of the 3D printed device. This study aimed to uncouple the role of the mesostructure and crystallinity in the final mechanical performance of carbon black/PLA 3D printed samples as a function of the printing parameters. A thermodynamic model was developed to determine the influence of printing parameters, such as nozzle diameter and layer height, in the final crystallinity. Specimens were mechanically tested under uniaxial tensile loads to determine the main deformation and failure mechanisms and results showed that samples were strongly influenced by the printing direction. Furthermore, the nozzle diameter played a significant role in the mesostructure and failure mechanisms, resulting in large differences in ductility for samples printed at raster direction 45 •. It was also found that the layer height had a strong influence on the temperature profile and the resulting crystallinity. The importance of crystallinity over porosity to improve the ductility of the material is a substantial contribution to the current state of the art. This work provides the basic fundamentals to manufacturing 3D printed components with tailored mechanical properties for specific loading conditions.