A versatile processing technique for fabricating epoxy nanocomposites with a high weight fraction of oxidized multi-walled carbon nanotubes is presented. Thin carbon nanotube based preforms were prepared through an oxidation-filtration protocol and then immersed in a pre-polymerized epoxy/curing agent solution in acetone. By adjusting the conditions for the oxidation of carbon nanotubes and the epoxy concentration in the as-prepared solution, high loading of graphitic nanostructures was obtained. Tensile tests indicated that the elastic modulus and strength of certain composites prepared by in situ polymerization as above were improved by 100% and 60%, respectively, compared to neat epoxy. In addition, the composite sheets showed comparable electrical conductivity values to the neat carbon nanotube paper. These results suggest that targeted chemical modification of the carbon nanotube surface is an effective way to enhance the electrical and mechanical properties of carbon nanotube-polymer composites.