Polyetherimide (PEI) has excellent mechanical and thermal properties, and exceptional fire resistance. Developing even broader multi-functionality in PEI/carbon nanotube (CNT) composites for industrial applications is an alluring but challenging goal, due to processing difficulties related to the high pressure and temperature needed to achieve effective flow for this polymer, and costly and complex treatments of the CNTs. Here we report the fabrication of PEI nanocomposite films using low-cost commercially functionalized multi-walled carbon nanotubes (MWNTs), and a simple and innovative process, achieving exceptional properties with only 0.5 wt% of MWNTs, including an increase in electrical conductivity of 12 orders of magnitude, accompanied by an unprecedented increase of 86 degrees C in thermal decomposition temperature (higher service temperature). Field emission scanning electron microscopy revealed a high degree of uniform dispersion among the MWNTs, superb polymer-MWNT interaction and formation of a spatially homogeneous nanotube network within the matrix. The enhancement in these properties suggests great potential use for this developed processing approach and the resulting nanocomposites for multi-functional coating or interfacing materials in aerospace and electronic industries.