Large-scale government digitalisation projects require collaborative approaches for their successful development and implementation. To shed light on these collaboration dynamics, the interplay between two integral parts of collaborative projects: the project rules, procedures, and structures (collectively known as institutional design) and the leaders managing these projects is studied. In doing so, the paper provides empirically grounded insights into the management of collaborative government digitalisation projects. Taking an institutional perspective, and the strategic-relational approach (SRA), we examine the extent to which institutional design constraints leadership behaviours, and under what conditions leaders begin to adjust and/or challenge these design features adding to the knowledge of managing digitalisation of government. To this end, we present a five-country case study of national digitalisation projects across Europe. Our results show that while clear institutional design features such as established rules, project structures, and standard operating procedures are essential at the initial stages of the projects, leaders' skills in understanding and tailoring these features are critical to handle project-related problems and moving forwards towards implementation. This underscores the importance of examining projects taking an SRA approach and the need to understand leadership behaviour in the context of the structures in which it is embedded.