Abstract: Background: The Children and Young People’s Improving Access to Psychological Therapies (CYP-IAPT) programme was introduced to transform Child and Adolescent Mental Health Services (CAMHS) across England. The programme comprised a set of principles that local CAMHS partnerships were expected to operationalise and embed with the aim of increasing access to services and improving the quality of care. This study explored how the implementation of the CYP-IAPT programme was executed and experienced by CAMHS professionals in the county of Cambridgeshire (UK), and the extent to which the CYP-IAPT principles were perceived to be successfully embedded into everyday practice. Methods: We analysed 275 documents relating to the CYP-IAPT programme issued between 2011 and 2015. We also conducted a thematic analysis of 20 qualitative interviews, undertaken at two time points, with professionals from three CAMHS teams in Cambridgeshire. Analysis was informed by implementation science frameworks. Results: Document analysis suggested that the CYP-IAPT programme was initially not clearly defined and lacked guidance on how to operationalise key programme principles and apply them in everyday practice. There was also a degree of programme evolution over time, which made it difficult for local stakeholders to understand the scope and aims of CYP-IAPT. Interviews with staff showed low coherent understanding of the programme, variable levels of investment among stakeholders and difficulties in collaborative working. Barriers and facilitators to programme implementation were identified at individual, service and strategic levels. These in turn impacted the local implementation efforts and sustainability of the programme in Cambridgeshire. Conclusions: We identified factors relating to programme design and national and local implementation planning, as well as features of inner and outer context, which impacted on the delivery and sustainability of the programme. These findings can be drawn upon to inform the development and delivery of other local and national quality improvement (QI) initiatives relating to children and young people’s mental health.