Composites of acid-functionalized multi-walled carbon nanotubes (MWCNTs) that reinforced poly(amide–imide) (PAI) were developed. The obtained composites containing 5, 10, and 15 wt% MWCNT–COOH exhibited a relatively good dispersion on the macroscopic scale. MWCNT/PAI composite films have been prepared by casting a solution of precursor polymer containing MWCNTs into a thin film, and its tensile properties were examined. The mechanical and thermal properties of the composites were investigated. The incorporation of MWCNTs–COOH improved the mechanical properties of the composites compared with that of neat PAI. An improvement in thermal properties of the MWCNT/PAI composites was also observed. Fourier transform infrared spectroscopy, powder x-ray diffraction, scanning and transmission electron microscopies were also used to evaluate the MWCNT/PAI composite system.