Water conflicts open windows of opportunity for grassroots movements to transform water systems. However, academic fields studying social movements in socio-environmental conflicts are not well equipped to deal with complexity, non-linear dynamics, and emergent properties. Therefore, these fields rarely engage with long-term complex social processes and dynamics leading to systemic socio-technical changes. Researching water conflicts driven by grassroots movements, we ask whether and how the latter can influence a socio-technical transition of a water management regime. Through an emblematic water conflict in Mexico, we analyse the grassroots movement's trajectory since the conflict´s inception by following the dynamic process of developing agency. Our findings show that throughout the conflict, the grassroots movement accumulated and mobilized diverse capitals to initiate water management strategies and practices that catalysed change in the water management regime by stalling the implementation of large infrastructures. Eventually, this led to the inception of a sustainable and just transition.