<p>The mechanical properties of components manufactured via fused filament fabrication (FFF) are generally lower than those of injection-molded parts due to the layer-by-layer deposition process and its reliance on interlayer diffusion. Fiber reinforcement offers a potential solution by improving strength, but its influence extends beyond mechanical performance. Despite extensive research, a significant gap remains in understanding interlayer diffusion, mechanical anisotropy, and the thermal and surface effects of fiber reinforcement in FFF composites. This study examines the effects of 10 wt% carbon fiber (CF) reinforcement in acrylonitrile butadiene styrene (ABS) on interlayer diffusion across varying layer thicknesses and its influence on mechanical properties. Increasing the layer thickness from 0.18 mm to 0.34 mm in pure ABS raises pore density from 3.35% to 11.20%, significantly reducing tensile strength. In contrast, ABS-CF composites exhibit a 41.98% average increase in tensile strength and reduced sensitivity to layer thickness variations. Anisotropy analysis, further incorporating raster angle and build orientation, shows a reduction in tensile strength and strain variation by 57.32% and 78.09%, respectively, indicating improved mechanical consistency. Furthermore, ABS-CF demonstrates enhanced thermal stability, with increased thermal expansion of deposited strands promoting interlayer diffusion. This not only improves mechanical performance but also results in distinct surface characteristics, where overall surface roughness remains comparable or lower, yet individual layer-level roughness rises by 57.30%. Additionally, surface hardness improves by 26.67%. However, these enhancements come with a transition from ductile to brittle failure, likely due to non-uniform fiber distribution and thermal mismatch between ABS and CF during fabrication.</p>