Recent decades have witnessed widespread agrarian transitions in mainland Southeast Asia. This paper examines how agrarian transitions are shaped by multiple drivers of change, and how these interwoven processes have triggered shifts in agricultural extension practices in three countries in the Lower Mekong Basin: Laos, Cambodia, and Vietnam. Drawing on interviews with experts working on the fields of agrarian studies and rural development, this paper argues that agrarian transitions not only put a strain on agricultural extension systems in responding to evolving needs, but they also stimulate the co-production of innovative agricultural extension models to address gaps left by the limited presence of extensionists. The study gains insight into challenges faced by extensionists, including a lack of resources, skills, and capacities to meet growing needs, which simultaneously urged them to excel in their work performance. The paper highlights the proactive role of ‘champions’ in orchestrating collective efforts towards the co-production of innovative agricultural extension models (e.g. Metkasekor), and the formulation of pluralistic extension platforms in enabling such ‘co-learning-to-act’ practices. By translating these insights into the broader contexts of agricultural and rural development in the Mekong region and beyond, this paper aims to make a two-fold contribution. First, it will assert how the ‘business-as-usual’ extension model has failed to adequately address emerging needs as a result of agrarian transitions. Second, it will provide pathways for the recognition and legitimisation of the pluralistic extension approach that fosters stakeholders’ co-learning and productive engagement in extension practices.