BACKGROUND/PROBLEM: Advance care planning (ACP) pragmatic trials are needed. PROPOSED SOLUTION: We determined key system-level activities to implement ACP interventions for a cluster-randomized pragmatic trial. We identified patients with serious illness from 50 primary care clinics across three University of California health systems using a validated algorithm. If patients lacked documented ACP within the last 3 years, they were eligible for an intervention: (Arm 1) an advance directive (AD); (Arm 2) AD + PREPAREforYourCare.org; (Arm 3) AD + PREPARE + lay health navigator outreach. Triggered by an appointment, we mailed and sent interventions through automated electronic health record (EHR) messaging. We collaborated with patients/caregivers, clinicians, payors, and national/health system leader advisors. We are currently finalizing 24 months follow-up data. OUTCOMES/METHODS: We used the Consolidated Framework for Implementation Research (CFIR) and Reach, Effectiveness, Adoption, Implementation, and Maintenance (RE-AIM) frameworks to track secular trends and implementation efforts. KEY MESSAGE/RESULTS: Required multisite, system-level activities: 1) obtaining leadership, legal/privacy, and EHR approvals; 2) standardizing ACP documentation; 3) providing clinician education; 3) validating an automated serious illness identification algorithm; 4) standardizing ACP messaging with input from over 100 key advisors; 5) monitoring secular trends (e.g., COVID); and 6) standardizing ACP workflows (e.g., scanned ADs). Of 8707 patients with serious illness, 6883 were eligible for an intervention. Across all arms, 99% received the mailed intervention, 78.3% had an active patient portal (64.2% opened intervention), and 90.5% of arm three patients (n = 2243) received navigator outreach. LESSONS LEARNED: Implementing a multisite health system-wide ACP program and pragmatic trial, with automated EHR-based cohort identification and intervention delivery, requires a high level of multidisciplinary key advisor engagement, standardization, and monitoring. These activities provide guidance for the implementation of other large-scale, population-based ACP efforts.