The increasing exposure to non-ionizing electromagnetic (EM) radiation driven by urbanization and digitalization has encouraged the development of building materials with EM shielding properties. This study investigates the potential of enhancing the electromagnetic shielding properties of cement mortars by incorporating multi-walled carbon nanotubes (MWCNT) in various dosages (1%, 3%, 6%, 9% and 10% by binder mass). The microstructural and mechanical effects of MWCNT addition, as well as their efficiency in reducing EM transmission in the frequency range of 1.5–10 GHz (covering LTE, 5G, WiFi, and radar systems), were analyzed. S₂₁ measurements were performed using a modified coaxial transmission line method with a vector network analyzer. Results show that increasing the MWCNT content enhances EM shielding effectiveness but simultaneously affects the mortar’s microstructure and mechanical properties. Higher MWCNT levels achieved the best EM shielding, with an improvement of up to 27.66 dB compared to ordinary mortar in the navigation radar frequency range. These findings confirm the potential of MWCNT-modified mortars for protecting buildings and sensitive infrastructure—such a hospitals, communication hubs, data centers and military facilities—from EM radiation.