Electromagnetic waves are produced in large volumes by the increasing number of electronic devices, and some devices must be shielded from these waves to prevent electromagnetic interference (EMI). Traditional metals are effective shielders but have limitations in modern electronics, so researchers have proposed the use of polymers containing conductive fillers. In the present study, waterborne poly(urethane-imide) (WPUI) composites reinforced with conductive carbon nanostructures (CNS) are developed for EMI shielding applications. NCO-terminated imide oligomers are successfully synthesized, so a solvent is not needed to incorporate imide structures into polyurethane. Additionally, a reactive nonionic dispersant is employed to prevent emissions from amine neutralizers. The proposed WPUI, which has high thermal stability and heat resistance, is thus environmentally friendly. The composites are experimentally evaluated through Fourier transform infrared spectroscopy, differential scanning calorimetry, and tensile stress–strain analysis and furthermore subjected to an EMI shielding test. Compared with polyurethane, the WPUI–CNS composite has considerably superior tensile strength and flexibility. Incorporating 5 wt% CNS into WPUI results in an EMI shielding effectiveness of 50 dB and does not negatively affect the WPUI's favorable physical properties, which is not the case for current alternatives. The optimal WPUI composite is found to have excellent performance under high-temperature conditions.