Project-based firms increasingly apply practices that are in line with relationship marketing (RM), which is a shift from individuality to mutuality, and servicedominant logic (SDL), which is a shift from a focus on goods exchange to an emphasis on service exchange. These firms also adopt value co-creation processes, benefiting from both RM and SDL views. However, their overall transition towards RM, SDL and/or both is not clearly understood. Therefore, the research aim is to explain how and why project-based firms have moved away from transitional marketing. A retrospective action research between 2002 and 2009 provides process data on how a single main contractor adapted their marketing approach over five publicly funded projects within the oil, gas and petrochemical industries. The findings show that the uniqueness and dynamics of projects necessitate the adoption of both SDL and RM, entitled the service-led relationship here. This transition occurs through services expansion, multi-level distributed interactions and process-oriented performance management in projects. The firm enabled the transition through the evolution of both institutional mechanisms such as contracts and organisational structure, and people attributes such as capabilities and culture.