There is a pressing need to prevent and address youth crime and violence owing to its prevalence, harms and cost to society. Interventions with proven effectiveness in doing this exist. Adopting and adapting them in new contexts is potentially cost-effective. However, more research is needed into how to make adaptations that enhance intervention implementation, effectiveness and maintenance in new settings. This article reports the pre-implementation adaptation work involved in transporting Becoming a Man (BAM) from the US to the UK. BAM is a selective school-based youth development program for 12–18 year-old boys that aims to improve school engagement and reduce interactions with the criminal justice system. We describe the nature of and rationale for adaptations and identify learning for future adaptation efforts. An adaptation team comprising the intervention developers, new providers and the evaluators met weekly for 10 weeks, applying a structured, pragmatic and evidence-informed approach to adapt the BAM curriculum and implementation process. Changes were informed by documentary analysis, group-based discussions and site visits. The group agreed 27 changes to the content of 17/30 lessons, at both surface (e.g., cultural references) and deep (key mechanisms or concepts) levels. Of 28 contextual factors considered, 15 discrepancies between the US and UK were identified and resolved (e.g., differences in staffing arrangements). Strengths of the process were the blend of expertise on the adaptation team in the program and local context, and constant reference to and ongoing refinement of the program theory of change. Limitations included the lack of involvement of school staff or students. Further research is needed into potential conflicts between stakeholder perspectives during adaptation and whose views to prioritise and when.