Competent delivery of interventions in child and youth social care is important, due to the direct effect on client outcomes. This is acknowledged in evidence-based interventions (EBI) when, post-training, continued support is available to ensure competent delivery of the intervention. In addition to EBI, practice-based interventions (PBI) are used in the Netherlands. The current paper discusses to what extent competent delivery of PBI can be influenced by introducing supervision for professionals. This study used a mixed-method design: (1) A small-n study consisting of six participants in a non-concurrent multiple baseline design (MBL). Professionals were asked to record conversations with clients during a baseline period (without supervision) and an intervention period (with supervision). Visual inspection, the non-overlap of all pairs (NAP), and the Combinatorial Inference Technique (CIT) scores were calculated. (2) Qualitative interviews with the six participants, two supervisors, and one lead supervisor focused on the feasibility of the supervision. Four of six professionals showed improvement in treatment fidelity or one of its sub-scales. Had all participants shown progress, this could have been interpreted as an indication that targeted support of professionals contributes to increasing treatment integrity. Interviews have shown that supervision increased the professionals’ enthusiasm, self-confidence, and awareness of working with the core components of the intervention. The study has shown that supervision can be created for PBI and that this stimulates professionals to work with the core components of the intervention. The heterogeneous findings on intervention fidelity can be the result of supervision being newly introduced.