Background Most implementations fail before the corresponding services are ever delivered. Measuring implementation process fidelity may reveal when and why these attempts fail. This knowledge is necessary to support the achievement of positive implementation milestones, such as delivering services to clients (program start-up) and competency in treatment delivery. The present study evaluates the extent to which implementation process fidelity at different implementation stages predicts achievement of those milestones. Methods Implementation process fidelity data—as measured by the Stages of Implementation Completion (SIC)—from 1287 implementing sites across 27 evidence-informed programs were examined in mixed effects regression models with sites nested within programs. Implementation process fidelity, as measured by the proportion of implementation activities completed during the three stages of the SIC Pre-Implementation phase and overall Pre-Implementation (Phase 1) and Implementati