Practical approaches are needed to take advantage of the nanometer-scale mechanical properties of carbon nanotubes (CNTs) at the macroscopic scale. This study was conducted to elucidate the salient factors that can maximize the mechanical properties of nanocomposites fabricated from commercially available CNT sheets. The CNT sheets were modified by stretching to improve CNT alignment and in situ polymerization using polyaniline (PANI), a π-conjugated conductive polymer, as a binder. The resulting CNT nanocomposites were subsequently postprocessed by hot pressing and/or high temperature treatment to carbonize the PANI as a means to improve mechanical properties. The PANI/CNT nanocomposites demonstrated significant improvement in mechanical properties compared to pristine CNT sheets. The highest specific tensile strength of PANI/stretched CNT nanocomposite was 484 MPa/(g/cm3), which was achieved in a sample with ∼42 wt % of PANI. This specimen was fabricated by in situ polymerization followed by hot pressing. The highest specific Young's modulus of 17.1 GPa/(g/cm3) was measured on a sample that was hot-pressed and carbonized. In addition, the highest DC-electrical conductivity of 621 S/cm was obtained on a sample prepared by in situ polymerization of PANI on a stretched CNT sheet.