High-entropy alloy films have shown excellent mechanical performance, but their hardness and wear resistance are still lower than those of conventional hard films such as TiN and CrN, which cannot meet the requirements of hard film applied under severe wear conditions. In this study, the (CuNiTiNbCr)CxNy high-entropy nanocomposite films were prepared by the double-target co-sputtering method. The effect of carbon content on the film morphology, phase structure, mechanical properties and tribological properties were investigated. With the introduction of carbon atoms in the film, the amorphous/carbonitride/amorphous carbon composite structure is formed. At 21.5 at.% carbon, the film hardness reaches the highest of 18.1 GPa. And at 37.6 at.% carbon, the film shows the highest toughness of 0.96 MPa m1/2 and exhibits the best tribological properties with a low friction coefficient of 0.27 and a wear rate of 1.0 × 10−6 mm3/N·m. The improvement in wear resistance is mainly attributed to the strong toughness of nanocomposite structures and the lubrication effect of amorphous carbon phase. The (CuNiTiNbCr)CxNy high-entropy nanocomposite films show good application prospect in harsh friction environments.