Background: A good transition from child to adult services remains a challenge for young disabled people (14-25) due to multiple organisations and professionals involved. Delineating what should happen is described in transition protocols/pathways, yet there is little evidence of their effectiveness, especially for young people for whom such protocols/pathways are intended to support. Aims and objectives: The aim was to understand the transition process, the contextual relationships, and the external and internal mechanisms facilitated by the intervention of a Transition Key Worker and how they may help create opportunities to achieve good outcomes for young people moving into adulthood. Methods: A Realist framework, as advocated by Pawson and Tilley, utilised an underpinning programme theory to unpick how protocols/pathways work to better manage the transition process and to evaluate how 14 sites implemented transition key working. A stakeholder workshop commenced the realist process to develop the mid-range theory and to identify what works, how it works and in what circumstances for young people. Thematic analysis was used to analyse 61 stakeholder interviews. I drew upon my ‘insider’ perspectives and reflexivity, which was a novel feature. Main Findings: Parents’ poor experiences of ‘pre-transitional’ support hindered preparation for their child’s future and determined their ability to think positively and let go of the past, which was not found in the literature or identified by Transition Key Workers. However, young people were not majorly concerned about the future. The mid-range theory areas of structuring the transition process, having support, planning well to make active decisions, with parents being ready for change within a robust governance and accountability framework were key indicators which determined what makes a successful transition. Transition Key Workers were regarded as an essential facilitator of the process. How to plan effectively and in detail with young people from was absent. Conclusion: The findings contribute by providing major insights into understanding what stakeholders considered to be the key elements of achieving successful transition. Determining a successful transition is individual and young people should not be coerced to fit into imposed systems, which limit their aspirations. The linear description of the transition process in current protocols/pathways does not fully represent the complexities where multiple organisations are involved. There is a need to set out in guidance how to plan with young people. The development of a new mid-range theory provides a significant contribution to inform practice in supporting young people to plan well for their futures.