This article discusses how cross-sector partnerships (CSPs) for sustainability manage the paradoxical tension between stakeholder inclusiveness and administrative efficiency. Drawing on qualitative data from a case study of a CSP focused on urban sustainability, we show how the inclusiveness–efficiency paradox unfolded throughout the studied collaboration. We discuss how the paradox reemerged in a different guise within each phase of the partnership and how three practices of paradox management helped actors to cope with the tension: “customized inviting” (during the formation phase), “sequential including” (during the preparation phase), and “tailored instructing” (during the implementation phase). On the basis of these findings, we argue that (a) the paradox reoccurred throughout the phases of the CSP because the three paradox management practices accentuated boundaries, thereby helping to resolve the paradox temporarily while at the same time creating grounds for the paradox to resurface, and (b) that the three paradox management practices can be theorized as a special type of boundary work that “plays up” relevant differences between actor groups and thereby ensures collaboration.