Small-group interventions are often used in schools to support the development of children's social skills. One such intervention, Lego®-Based Therapy (LBT), is increasingly popular internationally in schools. Although LBT has an emerging evidence-base, there is limited research exploring effective LBT implementation. An in-depth, exploratory multiple-case study design was used to explore the process of LBT assimilation in two English mainstream primary schools which were considered to have implemented LBT effectively and reported positive programme outcomes. Data collection was through structured observations and semi-structured interviews based upon Durlak and DuPre's (2008) established implementation framework. Transcripts were subjected to Hybrid inductive-deductive thematic analysis was used to identify themes which were triangulated with observation data. The data largely correlated with the Durlak and DuPre (2008) framework, identifying ley themes of: provider characteristics, characteristics of the intervention, organisational capacity and the need for a support system when implementing the intervention. Themes of facilitator factors and participant responsiveness emerged inductively, suggesting that a revised model of implementation is required for LBT. Recommendations include calling for EPs to use models of implementation when working with schools, as a cyclical, iterative process to promote positive outcomes and, therefore, cost-effectiveness of school investment in interventions. Limitations and future research are discussed.