Sustainability scholars increasingly recognize that policy mixes can positively impact socio-technical transitions. However, the temporal dimension of policy interventions remains under-researched, especially in the context of early transition dynamics that typically emerge in niche contexts. In this article, we explore how policy sequencing can play a key role in supporting the scaling-up of early-stage transition dynamics to drive wider system change. We contribute to transition research by proposing a process model for analyzing policy sequences by focusing on the interplay between policy instruments and institutional barriers. We conceptualize two ideal-type policy sequencing patterns - strategic and reactive - which we illustrate with empirical examples from early transition dynamics in the urban water sector of San Francisco (United States) and Sant Cugat del Vallès (Spain). Applying the process model to these case studies reveals how different sequences of policy (instrument) mixes can assist in overcoming institutional barriers, thus supporting transition trajectories.