Implementing new programs and practices is challenging, even when they are mandated. Implementation Facilitation (IF) strategies that focus on partnering with sites show promise for addressing these challenges. Our aim was to evaluate the effectiveness of an external/internal IF strategy within the context of a Department of Veterans Affairs (VA) mandate of Primary Care-Mental Health Integration (PC-MHI). This was a quasi-experimental, Hybrid Type III study. Generalized estimating equations assessed differences across sites. Patients and providers at seven VA primary care clinics receiving the IF intervention and national support and seven matched comparison clinics receiving national support only participated in the study. We used a highly partnered IF strategy incorporating evidence-based implementation interventions. We evaluated the IF strategy using VA administrative data and RE-AIM framework measures for two 6-month periods. Evaluation of RE-AIM measures from the first 6-month period indicated that PC patients at IF clinics had nine times the odds (OR=8.93, p<0.001) of also being seen in PC-MHI (Reach) compared to patients at non-IF clinics. PC providers at IF clinics had seven times the odds (OR=7.12, p=0.029) of referring patients to PC-MHI (Adoption) than providers at non-IF clinics, and a greater proportion of providers' patients at IF clinics were referred to PC-MHI (Adoption) compared to non-IF clinics (β=0.027, p<0.001). Compared to PC patients at non-IF sites, patients at IF clinics did not have lower odds (OR=1.34, p=0.232) of being referred for first-time mental health specialty clinic visits (Effectiveness), or higher odds (OR=1.90, p=0.350) of receiving same-day access (Implementation). Assessment of program sustainability (Maintenance) was conducted by repeating this analysis for a second 6-month time period. Maintenance analyses results were similar to the earlier period. The addition of a highly partnered IF strategy to national level support resulted in greater Reach and Adoption of the mandated PC-MHI initiative, thereby increasing patient access to VA mental health care.