Broadband and ultrathin electromagnetic interference (EMI)-shielding materials are crucial for efficient high-frequency data transmission in emerging technologies. MXenes are renowned for their outstanding electrical conductivity and EMI-shielding capability. While substituting nitrogen (N) for carbon (C) atoms in the conventional MXene structure is theoretically expected to enhance these properties, synthesis challenges have hindered progress. Here, it is demonstrated that TiCN T MXene films with optimized N content achieve a record-high electrical conductivity of 35 000 S cm and exceptional broadband EMI shielding across the X (8-12.4 GHz), K (26.5-40 GHz), and W (75-110 GHz) bands-outperforming all previously reported materials even at reduced thicknesses. By synthesizing a full series of high-stoichiometric TiAlCN MAX phases without intermediate phases, the impact of N substitution on the physical and electrical properties of TiCN T MXene flakes is systematically explored, achieving complete composition tunability in both dispersion and film forms. These findings position TiCN T MXenes as promising candidates for applications spanning from conventional lower-frequency domains to next-generation sub-THz electronics.