Abstract Background Although a complexity theory lens can help to understand national policy implementation at a local government level, application of this lens often remains metaphorical. We illustrate how complexity concepts (adaptation, feedback, emergence and co-evolution) can be used practically in research on policy implementation using a UK case study (Future in Mind; FiM), aimed at improving children’s access to mental health services in a municipality in northern England. Methods We re-analysed interview data with staff from local government, the National Health Service, schools, and the voluntary and community sector (n=31) involved in implementing FiM and coded this data in NVivo12 using complexity concepts as sensitising constructs. Findings We identified 5 feedback loops: two positives (1. flexing the training offer; 2. new skills, knowledge and behaviour by non-specialist staff) and three negatives (3. short termism, 4. free rider behaviour, 5. professional boundaries). These loops energised local adaptations of FiM by school and NHS staff, leading to system-level change (emergence), with the school system becoming more responsive to the mental wellbeing needs of children and young people, and shifts across systems (co-evolution), by developing joint values and language between schools and NHS. Conclusions We demonstrate the importance of positive and negative feedback loops for evidencing system-level change and shifts across systems. Our findings provide new insights into unintended consequences of policy implementation and starting positions for implementers to utilise feedback for generating impact. We argue for blurring of boundaries between implementation and evaluation in policy research to optimise the use of feedback.