Peer specialists (PS) in the Veterans Health Administration are veteran employees with lived mental health experience supporting others in recovery. While PS worked in traditional mental health settings for many years and demonstrated benefits to veteran engagement and satisfaction with health care, little is known about the best strategies to support implementation in novel, complex settings like primary care (PC). Implementation facilitation, which combined external facilitation plus national resources, was chosen to promote uptake when the VA MISSION Act of 2018 required the Veterans Health Administration to implement PS into PC. Using a mixed-methods, formative program evaluation approach, we examined the effects of implementation facilitation across two cohorts of 15 sites. Due to startup timing, Cohort 1 received minimal implementation facilitation during preimplementation compared with Cohort 2. Outcome measures included qualitative evaluation of facilitator notes, time to attain implementation milestones, and PS productivity and program fidelity (percent of total PS encounters in PC). Results suggested that overall productivity was similar across cohorts, but Cohort 2 demonstrated high program fidelity in Month 1 of implementation, while Cohort 1 required 12 months to attain similar fidelity. Qualitative findings indicated implementation facilitation that fostered team cohesion, clarified roles, supported supervisor training, and encouraged sites to guard against competing demands smoothed transitions for PS into PC. During preimplementation, implementation facilitation that focused on these findings resulted in the rapid attainment of program fidelity. Policymakers should consider the value of investing in preimplementation support for speeding the attainment of high-fidelity implementation of novel and complex practices such as PS in PC. (PsycInfo Database Record (c) 2024 APA, all rights reserved).