Abstract: A novel multicomponent high-Cr CoNi-based superalloy with superior comprehensive performance was prepared, and the evaluation of its high-temperature microstructural stability, oxidation resistance, and mechanical properties was conducted mainly using its cast polycrystalline alloy. The results disclosed that the morphology of the γ′ phase remained stable, and the coarsening rate was slow during the long-term aging at 900–1000°C. The activation energy for γ′ precipitate coarsening of alloy 9CoNi-Cr was (402 ± 51) kJ/mol, which is higher compared with those of CMSX-4 and some other Ni-based and Co-based superalloys. Importantly, there was no indication of the formation of topologically close-packed phases during this process. All these factors demonstrated the superior microstructural stability of the alloy. The mass gain of alloy 9CoNi-Cr was 0.6 mg/cm² after oxidation at 1000°C for 100 h, and the oxidation resistance was comparable to advanced Ni-based superalloys CMSX-4, which can be attributed to the formation of a continuous Al₂O₃ protective layer. Moreover, the compressive yield strength of this cast polycrystalline alloy at high temperatures is clearly higher than that of the conventional Ni-based cast superalloy and the compressive minimum creep rate at 950°C is comparable to that of the conventional Ni-based cast superalloy, demonstrating the alloy’s good mechanical properties at high temperature. This is partially because high Cr is beneficial in improving the γ and γ′ phase strengths of alloy 9CoNi-Cr.