In 2011, the newly elected Mayor 0f Seoul introduced administrative reforms that breathed new life into a series of grassroots initiatives. These high-profile reforms helped strengthen the connection between Seoul’s government, environment, and citizenry and spurred on changes that were pivotal to reorienting Seoul’s development trajectory. This chapter draws on sustainability transitions literature to trace the process enabling this reorientation. It highlights the critical role played by change agents such as the Mayor of Seoul in working at the boundary of ‘regimes’ and ‘niches’ to enable wider scale change; while citizen engagement can broaden the impact of grassroots innovation. It further underscores how key enablers in the transition process can help scale changes to and within the regime level. The chapter, however, draws a subtle but under-examined distinction between organisational and institutional change within the regime level. This within-level distinction could be the difference that gives rise to sustainability transitions research that changes Seoul as well as other cities in the future.