In serious game design, there is a growing trend towards involving players more substantially in the development process. However, several scholars and practitioners agree that still more research and empirical data are needed to guide such participatory prototyping processes. So far, less focus has been put on the influence of co-creation and co-design on participants in the process. This article explores the co-creation and co-design process in creating a serious game called “Mission BioHero”, with a focus on sustainability and the bioeconomy. The game aims to engage citizens in the bioeconomy, help them learn in a fun and interactive way, and aggregate their ideas to shape bioeconomy policies. This serious game was fully co-created and co-designed with citizens and stakeholders from eight regions in five European countries. This research investigates the nature and impact of co-creation and co-design on game development and its driving success factors, explores educational capacities of this participatory process on participants, and evaluates its impact on the co-designed serious game. The findings draw upon empirical data gathered from the co-creation and co-design phases, participants’ feedback, and external game players’ input. The study confirms the value of the participatory design process in serious games design and offers methodologies, tools and lessons learned for similar future processes.