Background <p>Digital mental health interventions (DMHIs) can expand access to mental health care, particularly in underserved regions. Despite this, real-world implementation is complex and few studies have investigated the factors that shape it. We provide insights from Help@Hand, a five-year implementation project deploying DMHIs within public systems across counties and cities (i.e. sites) in California. This study examined how these sites implemented DMHIs within their local systems and identified factors that facilitated or prevented success.</p> Methods <p>Data were collected from project leads at participating sites (<i>N</i> = 12) through four waves of surveys and two rounds of interviews. Surveys and interviews were informed by the Consolidated Framework for Implementation Research (CFIR) and were iteratively adapted across waves to capture implementation experiences at different points in the project. Interviews and surveys were analyzed using a sequential mixed-methods design. Reflexive thematic analysis and descriptive survey statistics were used to examine implementation strategies, challenges, and adaptations.</p> Results <p>Integrating four surveys and two interviews, we identified seven themes whose prominence differed across project stages: (1) vendors supplemented internal expertise but offered limited customization and data access; (2) workforce challenges were common where all sites reported early labor shortages that resolved at later stages; (3) transitions to sustainment proved challenging as no sites obtained new funding at the final survey; (4) community outreach shifted toward targeted, multimodal approaches; (5) centralized project management aided contracting and coordination but introduced delays to approvals; (6) cross-site collaboration declined as the project progressed; (7) digital equity remained challenging despite greater outreach efforts reported by final project stages.</p> Conclusions <p>This study provides several insights into the implementation of DMHIs within public behavioral health systems: hybrid centralized-local governance models can support coordination but may add layers of decision-making; the same determinants may be differentially experienced across sites depending on their local context; sustainment can be difficult, even with planning, where funding is difficult to obtain. We also demonstrate that the CFIR has utility in structuring implementation evaluations across multiple sites and project stages. Future multi-site DMHI initiatives should pair centralized coordination with flexible local pathways, account for differential site capacity, resource sustainment from the outset, and use frameworks such as the CFIR to structure their implementation evaluation.</p>