Manufacturing flexibility is often viewed as a strategic capability that enables firms to more effectively meet heterogeneous market demands arising, in part, from increased product proliferation. However, recent studies suggest that the operational challenges associated with meeting this objective may be heavily dependent not only upon a firm's internal modification, mix, and new product flexibilities, but also upon the flexibility of its inbound and outbound supply chain partners. Drawing upon the theory of Complementarity, we examine if simultaneous utilization of both internal and external flexibilities does in fact create synergies that can improve a firm's delivery performance. Based on a sample of 158 U.S. manufacturing plants, we find that the extent to which performance enhancing synergies are generated is primarily dependent upon the type of internal flexibility that is paired with supply chain flexibilities. Additionally, we find that when synergies do exist, external supplier and logistics flexibilities generally tend to enhance the scope of flexible response, while internal flexibilities generally tend to enhance the achievability of a flexible response. Taken together, our findings suggest that the ability of firms to actually reap the synergistic benefits of an integrated system of supply chain flexibility is much more complex and nuanced than previously believed or expected.