Two-dimensional (2D) MXene materials have potential applications in electromagnetic pollution elimination and energy conversion fields owing to their high carrier mobility, specific surface area and unique layered features. Here, we demonstrated a controlled self-assembly strategy to construct multifunctional nano/microstructured 2D Ti3CNTx MXene-based composites for efficient electromagnetic attenuation and Joule energy conversion application. The multiple compositions and novel architecture endowed the layered porous Ti3CNTx/Ni@C composites with enhanced electromagnetic wave absorption and EMI shielding performances. Notably, the optimized nanocomposites achieved a high-performance electromagnetic wave absorption with a low loading (8 wt%), a broad effective absorption bandwidth (5.4 GHz) and a strong absorption (−65.7 dB) at a thickness of only 1.5 mm. The high electromagnetic shielding efficiency reached 66.7 dB. The underlying mechanism of electromagnetic shielding and absorption was the synergistic effects among electronic transport, abundant interfacial/dipolar polarization, strong magnetic coupling and multiple reflections in the layered porous structure. In addition, the efficient Joule energy conversion with favorable flame-retardant and high infrared shielding versatility broadened the practical application range of Ti3CNTx/Ni@C composites in harsh environments. Therefore, this assembly strategy inspires the construction and application of the 2D Mxene-based composites to promote the development of energy conversion and green electromagnetic environments.