In this longitudinal study, we explore the co-evolution of the cognitive complexity of the CEO of Puma, Jochen Zeitz, and his view and initiatives on sustainability. Our purpose was to explore how the changes in a leader’s mindset relate to his/her views and actions on sustainability. In contrast to previous studies, we adopt an in-depth longitudinal case study approach to capture the role of leaders’ cognitive complexity in the context of corporate sustainability. By understanding the cognitive development of Zeitz as leader of Puma, we provide an important step toward understanding the co-evolution of leaders’ cognitive complexity and proactive corporate sustainability initiatives over time. Our findings show that as he developed a more complex mindset that also included non-business lenses, Zeitz developed a more inclusive understanding of sustainability and adopted proactive initiatives that went beyond business-as-usual. Our study also demonstrates that a longitudinal perspective can offer valuable insights for a better understanding of how individuals and their interactions affect and are affected by an organization’s strategies and performance, in corporate sustainability and beyond.