The effect of extreme and negative Poisson’s ratio(auxetic) behaviour from non-reconstructed defective (NRD) carbon nanotubes on the mechanical properties of nanocomposites is demonstrated. NRD carbon nanotubes (CNTs) produced by electronic or ion irradiation are metastable structures, which however can exist at low temperatures or when surrounded by an external medium (e.g. CNT nanoropes of matrix in a composite). The defective nanotubes were modelled using an equivalent atomistic-continuum formulation for the mechanical properties of the C–C bond that provides both the effective mechanical properties of the atomic link and the thickness of the bond itself. The defective CNTs are modelled as truss-type finite elements, with the defective atoms selected using a random Latin hypercube algorithm. The mechanical properties (Young’s modulus, shear and bulk modulus, as well as Poisson’s ratios) are obtained using uniaxial tensile loading simulations. The defective nanotubes not only exhibit negative Poisson’s ratio (PR), but also extreme positive PR values based on the percentage of defective atoms, radius and aspect ratio of the tubes. Hashin-Shritkman bounds are used to calculate the effective properties of nanocomposites with different matrix combinations and CNT inclusions. Based on the properties of the defective CNTs, extreme values of 20 bulk and shear modulus for the nanocomposites are recorded.