Implementing and sustaining high-quality early intervention practices depends on access to an evidence-based professional development (PD) model that fits the unique features of the system, providers, and families served. This paper describes and evaluates a 10-month PD sequence for EI providers on Family Guided Routines Based Intervention (FGRBI) conducted across an 8-year period within a U.S. state of Iowa Part C program. The PD sequence was led first by external coaches (ECs) and then by trained peer internal coaches (ICs). Adherence and quality fidelity measures in 249 providers showed positive changes in FGRBI implementation that were maintained as PD transitioned to ICs for sustainability. This paper extends the evidence supporting the use of a comprehensive PD model, including ongoing coaching for EI providers making complex changes, such as caregiver coaching. Future research should examine the integration of other EI processes (i.e., assessment or progress monitoring) within the PD in addition to caregiver coaching.