Ultrathin, lightweight, and flexible electromagnetic interference (EMI) shielding materials are urgently demanded to address EM radiation pollution. Efficient design to utilize the shields' microstructures is crucial yet remains highly challenging for maximum EMI shielding effectiveness (SE) while minimizing material consumption. Herein, novel cellular membranes are designed based on a facile polydopamine‐assisted metal (copper or silver) deposition on electrospun polymer nanofibers. The membranes can efficiently exploit the high‐conjunction cellular structures of metal and polymer nanofibers, and their interactions for excellent electrical conductivity, mechanical flexibility, and ultrahigh EMI shielding performance. EMI SE reaches more than 53 dB in an ultra‐broadband frequency range at a membrane thickness of merely 2.5 µm and a density of 1.6 g cm−3, and an SE of 44.7 dB is accomplished at the lowest thickness of 1.2 µm. The normalized specific SE is up to 232 860 dB cm2 g−1, significantly surpassing that of other shielding materials ever reported. More, integrated functionalities are discovered in the membrane, such as antibacterial, waterproof properties, excellent air permeability, high resistance to mechanical deformations and low‐voltage uniform heating performance, offering strong potential for applications in aerospace and portable and wearable smart electronics. A designed scalable, ultrathin, and flexible cellular membrane, composed of high‐conjunction metal‐wrapped polymer nanofibers, exhibits excellent electromagnetic interference shielding performance. Moreover, the membranes effectively integrate multifunctionalities such as antibacterial and waterproof properties, air permeability, and low‐voltage uniform heating performance, offering strong potential for applications in aerospace and portable and wearable smart electronics.