This study presents a novel framework for evaluating the long-term resiliency of flood management strategies in complex nonurban-urban watersheds, with a focus on the synergy between grey and green infrastructure. The methodology involves simulating hydrological conditions of both upstream and downstream areas using the SWMM model, followed by a two-stage scenario generation approach. The first stage optimizes the implementation of detention dams in the upstream nonurban region, while the second stage focuses on optimizing Green Infrastructure (GI) in the downstream urban area. By integrating grey and green flood control measures, a total of 2500 combined scenarios were generated and tested under extreme hurricane conditions to assess their resilience and economic feasibility. The results highlight that higher investments in both detention dams and GI significantly enhance system resiliency, and leads to faster recovery after flood events. Strategies with greater infrastructure investment maintained higher performance throughout sequential flood events, and exhibited smaller reductions in effectiveness during peak events. Conversely, lower-cost strategies experienced greater performance degradation. Then, using resilience performance threshold, we identified high-performing strategies and employed the Condorcet method to select the optimal scenario that balances long-term resiliency with cost-effectiveness. The selected strategy reduced peak flow at the urban area’s entrance by 85.7% and decreased local urban flood volume by 65.5%. Our research underscores the critical role of integrated grey-green infrastructure in achieving long-term flood resilience. Also, this framework provides decision-makers with actionable insights for designing cost-effective, high-resilience flood management strategies that consider both economic and community benefits.