Background In 2011, the Children and Young People’s Improving Access to Psychological Therapies (CYP-IAPT) programme was introduced nationally to improve outcomes and experience of care for children and young people. The aim was to support services to embed key CYP-IAPT principles into everyday clinical practice so as to deliver evidence-based practice, demonstrate accountability through outcome monitoring, increase access to effective services and engage children and young people to participate in decisions about treatment and services. The current study explored the process of implementation and the experiences of professionals in specialist mental health settings in Cambridgeshire. Methods i) An audit and analysis of national and local documents issued between 2011 and 2015 and mapped along a five-year timeline; ii) Thematic analysis of in-depth interviews with 20 staff working in three specialist mental health settings in the Cambridgeshire and Peterborough Foundation Trust, conducted at two time points along the implementation trajectory. Results While there was high investment from frontline staff, there was a lack of clarity about the scope and aims of the CYP-IAPT programme during the early implementation phase. Programme developers gave more emphasis to some principles than others and there was a variation in perceptions of the extent to which principles were embedded. The creation of dedicated staff posts were a key driver to implementing and sustaining the CYP-IAPT model at a local level; specialist training and enhanced supervision facilitated evidence-based practice and outcome monitoring. Barriers to implementation included inadequate and inflexible IT systems, time limited funding and a lack of support from senior management. Organisational differences between partner agencies led to ineffectual collaborative working and high staff turnover prevented knowledge continuity. Conclusion This study provides valuable insights into local implementation of a complex national quality improvement programme. Recommendations will inform other local quality improvement initiatives relating to children and young people’s mental health.