High-performance elastic and ultrathin electric heating materials were prepared by forced impregnation of polydimethylsiloxane (PDMS) matrix into multiwalled carbon nanotube (MWCNT) nanopaper (NP) by using an efficient two-step ultrasonication treatment technique. Morphological feature, stretchability, electrical property, and electric heating behavior of the MWCNT/PDMS nanocomposite film (NC film) were investigated. Scanning electron microscope images confirmed that the MWCNTs were well impregnated in PDMS matrix with no aggregations. Relatively high MWCNT content (>10wt%), which results from the compact stack structure within NPs, in the MWCNT/PDMS NC film guaranteed the high thermal conductivity and excellent electric heating behaviors with significantly improved uniformity. The small thermal capacitance of PDMS matrix and high thermal conductivity of MWCNTs enabled temperature response rapidity at constant applied voltages. The superior and stable heating-cooling cyclic performances (temperature responsiveness, electric power efficiency, and steady-state maximum temperature) of MWCNT/PDMS NC film made it an ideal option for actual electric heating applications.