The pressing need for sustainability transitions requires not only new project outcomes, but also new ways of leading and organizing projects. The study aims to examine how circularity, understood as the reuse and recycling of materials, challenges the conventional project life cycle by changing processes, roles, and governance. A case of a nature-based park constructed with circular materials was conducted using interviews, observations, and site visits. The results show that circularity reshapes project leadership in three key ways. First, circular material sourcing requires an iterative and adaptive project process, which extends the front-end and overlaps with delivery. This disrupts linear, stage-gated models and introduces ongoing decision-making and flexible planning throughout the lifecycle. Second, the changing process reconfigures project roles: project managers take on expanded and evolving leadership, including facilitating collaboration, managing uncertainty, and communicating sustainability values across phases. Third, the findings underscore the need for adaptive governance, including engaged clients and flexible contracting, to support circular practices. These insights extend existing research on sustainability in and by projects by showing how circularity is not merely a design principle but an organizing logic that restructures internal processes, leadership roles, and governance arrangements. The study advances theory on how circularity functions not only as a sustainability goal but also as a transformative force in project organizing and leadership.