The development of sustainable, high-performance composite materials has led to an increasing interest in natural fiber-reinforced polymers enhanced with nanofillers. This study investigates the effect of graphene nanoplatelets (GnPs) on the interfacial and mechanical properties of flax fiber-reinforced epoxy composites. The GnPs were added at concentrations ranging from 0 wt.% to 6 wt.% while maintaining a constant flax fiber volume fraction of 25%. The mechanical performance of Flax/Epoxy/GnPs (F/E/GnPs) composites was assessed alongside their corresponding Epoxy/GnPs (E/GnPs) matrix composites. The interlaminar shear strength (ILSS) of the F/E/GnPs composites was measured using the short-beam bending method. Results showed that the GnPs addition decreased the fracture strength, fracture strain, and toughness in the E/GnPs matrix composites but significantly improved the stiffness. Incorporating flax fiber into these matrix composites enhanced the mechanical strength across all compositions. Notably, the F/E/1.5GnPs composite demonstrated optimal performance, with a 166% increase in the fracture strength and a 500% increase in the modulus of elasticity compared to E/1.5GnPs, alongside an 18% improvement in ILSS relative to neat epoxy. These improvements were attributed to the better interfacial adhesion and stress transfer caused by the uniform GnPs dispersion. However, increasing the GnPs content beyond 1.5 wt.% led to agglomeration and higher matrix viscosity, which weakened the fiber wetting and reduced interfacial bonding. SEM analysis of E/GnPs matrix composites revealed a more brittle fracture behaviour with a higher GnPs content. In the F/E/GnPs composites, excessive GnPs content further reduced the interfacial cohesion between flax fibers and the epoxy resin, likely caused by limited wetting during the fabrication process. These findings highlight the importance of optimising the GnPs content to balance the strength, adhesion, and processability in natural fiber composites.