In this manuscript, we describe the influence of -COOH functionalized carbon nanotubes, CNTs, on the thermo-mechanical and dielectric behaviors of PMMA composites containing montmorillonite, MMT. The composites were prepared through a thermally activated, in-situ, radical polymerization utilizing benzoyl peroxide as initiator. MMT was used in fabrication of the composite for its ability to generate viscous monomer slurries, which played a critical role in dispersing and suspending the CNTs. Although the addition of MMT and CNTs was found to improve thermal stability of PMMA, the glass transition temperature of the polymer was significantly reduced by these two fillers. Additionally, the composites containing CNTs exhibited significantly increased Young's modulus values relative to the PMMA-MMT composites, after initial stretching to align the CNTs in the direction of the applied stress. However, the gradual addition of CNTs to the polymeric systems resulted in increased dielectric constants, suggesting that these systems are possible candidates for the dielectrics in structural capacitors.