Rangeland degradation due to unsustainable land use is a global concern. In Iceland, livestock overgrazing has contributed to significant degradation and desertification, yet policy and management efforts to align agricultural practices with sustainability goals have fallen short of intended social and ecological outcomes. This study examines four episodes of boundary work in Icelandic rangeland governance between 1997 and 2021, analysing how actor relationships, shifting governance structures, and knowledge integration influenced sustainability outcomes. Findings reveal an initial period of openness, where scientists and policymakers successfully mobilised farmers toward sustainability-oriented actions. Limited knowledge co-production and frequent institutional restructuring, however, disrupted trust-building efforts, weakened legitimacy, and led farmers to increasingly question sustainability criteria, which resulted in policy resistance and contested governance arrangements. Ecological assessments became sources of conflict, reflecting divergent stakeholder understandings of sustain-ability and knowledge credibility. The episode-based analysis of boundary work used within this study demonstrated how credibility, legitimacy, and salience of knowledge evolve over time, shaping governance trajectories. The study highlights the need for stable yet adaptive governance mechanisms that embed participatory knowledge co-production from the outset, explicitly address power-dynamics, and ensure transparency, accountability, and continuous social-ecological monitoring and evaluation. It also demonstrates that participation alone does not guarantee sustainability transformation unless it challenges existing power structures and fosters institutional adaptation. Strengthening scientists' capacities for transdisciplinary research and knowledge brokerage is essential for promoting more inclusive and effective sustainability governance.