In this work, ‘trapped’ carbon nanotubes (CNTs) by polyacrylonitrile (PAN) are obtained through a developed non-solvent induced phase separation process. The ‘trapped’ CNTs were shown to possess a certain bundle size distribution and strong polymer-CNT interactions, which are beneficial toward the processing of superior polymer-based composites. These subset of CNTs were separated from the rest through a two-step filtration method, where a bilayer buckypaper-like composite films can be formed consisting of a CNT-rich layer (CR-L) and a polymer-rich layer (PR-L). Electrical properties were measured to show a highly conductive CNT network in the PR-L. Mechanical testing and analysis were also performed to reveal mechanical properties of resultant composite films. Lamination theory was used to resolve effective mechanical contributions from the CNT network within PR-L. These analysis results indicate that CNTs exhibit different reinforcement efficiency depending on the CR-L or PR-L regions in the composite. It is shown that effective Young’s modulus contribution of CNT in the PR-L region is substantially increased when compared with CR-L Region. Therefore, it is important for composite preparation to control the direct blending of the CNT with the PAN based on their inherent/natural interactive relationship.