Within this study, the impregnation behavior and resulting mechanical properties of unidirectional flax fiber-reinforced polyamide 11 biocomposites were investigated. Therefore, different grades of bio-based polyamide 11 have been evaluated regarding their eligibility as composite matrix material. The production of the unidirectional flax fiber-reinforced biocomposites was investigated using a continuous film-stacking method. It was found that the flow behavior of the polyamide 11 matrix polymer significantly affected the impregnation quality and the resulting mechanical properties as tested by tensile and bending tests. A lower shear viscosity and stronger shear thinning behavior led to better impregnation, a 15% higher stiffness, and 18% higher strength. This was also analyzed with morphological analysis by scanning electron microscopy. Additionally, the effect of the fiber volume content of the flax fibers on the mechanical properties was tested, showing a positive correlation between the fiber content and the resulting stiffness and strength, leading to an increase of 48% and 55%, respectively. In result, a maximum Young's modulus of 16.9 GPa and tensile strength of 175 MPa at a fiber volume content of 33% was achieved. Thus, the unidirectional flax fiber-reinforced polyamide 11 biocomposites investigated can be a sustainable construction material for lightweight applications, e.g., in the automotive industry.