We developed the ACP+ intervention to support nursing home staff with implementation of advance care planning. While ACP+ was found to improve staff's self-efficacy, it did not change their knowledge about advance care planning. To describe the level of implementation, mechanisms of impact, and contextual factors. Process evaluation embedded in a cluster randomized controlled trial in nursing homes (NCT03521206). Throughout and immediately following the 8-month ACP+ implementation, we collected weekly diaries, post-training surveys, attendance records, facility data, and conducted interviews (n = 32). We applied descriptive statistics and thematic analysis. Management, staff, and ACP+ trainers in seven intervention homes. Although most participants reported they valued ACP+, 33% of eligible staff across nursing homes attended training (range: 6%-69%) and only a few reported they felt ready to engage in actual care planning conversations. Half of all nursing homes adapted parts of the intervention (e.g., more/fewer/shortened training; assigning a coordinating role for practical management). Enough time to consolidate skills, and management support were key for staff engagement in advance care planning, and limited time and staff shortages were significant barriers. Staff reported increased awareness of the importance of advance care planning and felt there was a more systematic way of organizing advance care planning. There was limited staff engagement. Management ownership, clear roles, and collaborative practices may enhance nursing home advance care planning. Accessible and ongoing training for all staff, and ample practical learning opportunities are needed.