Multi-wall carbon nanotubes (MWCNTs) with lengths of 0.65–1.3 mm were used to fabricate aligned and continuous MWCNT/bismaleimide composites. We found that longer CNTs resulted in higher thermal and electrical conductivities of the composites. The tensile strength and Young’s modulus, however, exhibited no CNT length dependency. Investigation of the CNT morphology by transmission electron microscopy revealed that the average nanotube diameter and wall number also increased with the CNT length, while the aspect ratio remained nearly unchanged. The structural changes significantly affected the phonon and electron transport in the composite structure, but the interplay of increased CNT length and diameter led to no appreciable change in the mechanical properties of the composites.