Mechanically robust, magnetic nylon-6 nanocomposites reinforced by one-dimensional (1D) carbon nanotube (CNT)-two-dimensional (2D) clay nanoplatelet hybrids have been prepared using a simple melt-compounding technique. The direct iron-catalyzed chemical vapor deposition (CVD) growth of multiwalled CNTs utilizes iron oxide-immobilized clay nanoplatelets as substrates, carrying out in situ intercalation and exfoliation of clay nanoplatelets. By using such a hybridization and coexfoliation method, the as-obtained heterostructured hybrids used without any purification are demonstrated to be ideal and excellent nanofillers for mechanical reinforcement for fabricating nylon-6 nanocomposites, due to their homogeneous dispersion and strong interfacial interaction with the polymer matrix. The nucleation sites provided by the nanohybrids seem to be favorable to the formation of thermodynamically stable α-phase crystals of nylon-6 with much higher stiffness and hardness than γ-form of nylon-6, namely, a silicate-induced crystal transformation from the α-form to the γ-form of nylon-6 was greatly inhibited or "shielded" by the CNT-wrapped clay nanoplatelets. Furthermore, the nanostructured CNT-clay hybrid heterostructures containing residual iron oxide nanoparticles show novel magnetic properties in both bulk solids and polymer nanocomposites. Therefore, this can be probably developed into a facile and practical method to fabricate polymer nanocomposites with high performance and multifunctionality.