Sustainability transitions demand not only new strategies and governance structures but fundamentally new ways of seeing, relating, and acting within complex systems. Yet the mindset and posture shifts that make transition-oriented work possible are rarely treated as objects of intentional design; they are assumed rather than cultivated. This paper introduces the Transdisciplinary Sustainability Transitions Framework (TSTF), a staged, experience-based process architecture that treats cognitive and relational transformation as primary design intentions rather than incidental outcomes. It sequences participatory engagement to enable identifiable experiential thresholds, moving from fragmented awareness through discomfort and reframing toward distributed agency, across personal, relational, and systemic scales. Drawing on three cross-sector organizational case studies, the paper demonstrates how deliberate process design can scaffold the inner shifts that transition-oriented work requires. Findings suggest that in an era of accelerating complexity, the critical bottleneck is no longer the quality of sustainability strategies but the transformation of the people who must enact them, and that this transformation can be deliberately facilitated through well-structured, scalable, practice-oriented process design.