Electromagnetic interference (EMI) generated by electronic devices is disruptive to surrounding electronic components and deemed harmful to human beings. As high-speed, miniaturized mobile devices become ubiquitous, developing lightweight, thin, and flexible electromagnetic shielding materials (ESMs) to replace the common metallic ones is of utmost importance. Here we report the development of a high-efficiency standalone ESM made of a self-standing CNT mat (CNTM). This textile is electrically conductive (∼50 × 10³ S m⁻¹) and can be post-treated by chemical oxidation or thin copper metalization, increasing the conductivity by ∼10 and ∼1000 times, respectively. CNTMs were tested for shielding effectiveness (SE) in an ultra-wide bandwidth of 30 kHz–70 GHz. Maximal SE of >120 dB was measured for a 30 g m⁻² CNTM (<100 μm thickness) at 70 GHz. SE normalized by thickness (SE/t) reached a value of >20,000 dB cm⁻¹ for an HCl-oxidized CNTM. Both SE values are the highest values for non-metal-based ESMs. The CNTMs' experimental EMI shielding behavior corresponds to Schelkunoff's theory which confirms that the CNTM SE is dominated by conductivity-dependent reflection at lower frequencies (<1 GHz) and thickness-dependent absorption at higher frequencies. CNTMs (20 g m⁻²) were utilized as EMI gaskets and in composite EMI enclosures, tested at 10–50 GHz and 50 MHz-90 GHz, respectively. EMI CNT gaskets reached an SE of 120.9 dB at 50 GHz, surpassing the performance of high-end commercial gaskets (600 g m⁻²). EMI composite boxes laminated with CNTMs reached an SE of 119.9 dB at 90 GHz, outperforming the non-CNT laminated reference.