The increasing need for electromagnetic interference (EMI) shielding of electronics in cold environments such as those in aircraft, space exploration, and wearable heaters to avoid hazardous icing conditions or hypothermia requires the development of thin and lightweight EMI shielding materials preferably by absorbing rather than reflecting electromagnetic (EM) waves while also generating heat through energy-efficient electrothermal conversion. However, it is challenging to achieve absorption-dominant EMI shielding and energy-efficient electrothermal heating simultaneously in a thin and lightweight structure. Here, we develop a heterogeneous composite film comprising a porous multi-walled carbon nanotubes (MWCNTs)/bacterial cellulose (BC) film and an aligned MXene/Ag nanowires (NWs) backing via a sequential vacuum filtration process. The porous film contains random conductive networks of MWCNTs with moderate conductivity while the aligned MXene sheets atop Ag NWs network affords high conductivity in the backing, giving rise to graded electrical conductivity for absorption-dominant EMI shielding. The increasing Ag NW coverage leads to significantly increased electrical conductivity without increasing the EM wave reflection as well as the density and thickness of the film, yielding excellent specific EMI shielding effectiveness (> 8500 dB/(g·cm²)), low driving voltage for energy-efficient electrothermal heating (163 °C at 2.5 V), and fast response time (60 s) at a low areal density of 0.015 mg/cm². Combining EMI shielding and electrothermal heating, the heterogeneous film developed here are promising contenders for the protection of electronic equipment in low-temperature environment.