<p>Carbon nanotubes (CNTs), particularly when surface-functionalized, offer great potential for enhancing the mechanical and swelling resistance properties of natural rubber/ethylene-propylene-diene monomer (NR–EPDM) composites. In this work, pristine CNTs and 1-hexadecyl-3-methylimidazolium bromide-functionalized CNTs (HDMIM-Br@CNTs) were incorporated into the NR–EPDM matrix at loadings up to 8 phr using conventional melt mixing. The curing characteristics were analyzed using an oscillating disc rheometer, while the composites were evaluated for tensile strength, hardness, rebound resilience, abrasion resistance, compression set, crosslink density, and swelling resistance. Increasing CNT loading elevated cure torque and reduced curing time, indicating enhanced filler–rubber interaction. For unmodified CNT-filled composites, the tensile strength, 100% modulus, and tear strength improved by 25%, 26%, and 22%, respectively, with optimum performance at 6 phr. Beyond this level, agglomeration led to reduced mechanical strength and elongation at break. In contrast, HDMIM-Br@CNT-filled composites exhibited superior performance, with peak improvements of 32% in tensile strength, 35% in modulus, and 25% in tear strength compared to the unfilled blend. Swelling resistance also improved notably due to enhanced crosslink density and restricted polymer mobility. The findings confirm that HDMIM-Br functionalization enhances CNT dispersion and interfacial adhesion, thereby significantly improving the overall performance of NR–EPDM composites.</p>