The importance of program implementation in achieving desired outcomes is well-documented, but there remains a need for concrete guidance on how to achieve fidelity to evidence-based models within dynamic local contexts. Housing First (HF), an evidence-based model for people experiencing homelessness and mental illness, provides an important test-case for such guidance; it targets a uniquely underserved subpopulation with complex needs, and is delivered by practitioners with varying knowledge and skill levels. Scientific evidence affirms HF’s effectiveness, but its rapid dissemination has outpaced the ability to monitor not only whether it is being implemented with fidelity, but also how this can be achieved within variable local contexts and challenges. This qualitative study contributes to this need by capturing insights from practitioners on implementation challenges and specific strategies developed to overcome them. Findings reinforce the importance of developing HF-specific implementation guidelines, and of engaging relevant stakeholders throughout all phases of that development.