BackgroundWhen transitioning to adulthood, youth with disabilities and their families face many service gaps. Successful inter-agency collaborations can promote family-centred, inclusive transition support amenable to personal choice and health conditions. This paper reports the 3-year co-design process of an innovative transition service that links a pediatric hospital and adult service agencies and addresses key areas of transition preparedness with joint accountability.MethodsA team of pediatric rehabilitation professionals, adult service providers, young adults with disabilities and their families, and researchers engaged in a co-design process over three years. Following a design thinking (DT) framework, the team went through an iterative process of Empathize. Define, Ideation, Prototyping, and Testing phases. The trial-and-error process allowed for deeper reflection and an opportunity to pivot the design.ResultsThe co-design yielded Transitions Pop-ups, a nimble service model that can “pop up” at critical times and places to meet clients’ urgent and emergent transition-related needs. Two pilot sessions were conducted at the testing phase with adult service agencies. The final model included five key elements: (1) community partnership; (2) targeted information sharing; (3) peer mentoring; (4) action (on-the-spot completion of a key transition task/activity such as submitting an adult funding application); and (5) warm handover.ConclusionThe co-design process highlighted the importance of open communication and iterative prototype testing as a means for trialing new ideas and clarifying the intent of the project. The DT framework optimally facilitated the co-development of a contextually relevant and sustainable service model for pediatric rehabilitation clients and families.