Objectives: Although Housing First (HF) is a popular evidence-based intervention for persons experiencing homelessness and mental illness, research exploring its sustainability over time is scant. This mixed methods study captures practitioners' perspectives on key shifts in implementation of Housing First in a large urban center, and factors affecting fidelity to the model over time. Methods: This study uses fidelity assessments, at baseline and 18-months later, completed as part of the largest field trial of Housing First to date, and qualitative data from 28 front-line service providers and their managers. Results: Our findings reveal how the shifting landscape of implementation morphed some early troublespots into persistent challenges and exposed key factors facilitating fidelity to the model in a dynamic context. Conclusions: This study supports the importance of gathering provider perspectives over time to optimize the fit between the model and its changing local contexts and ensure maximal benefit of the intervention.