Interventions to improve organizations are common in both practice and research, but relatively little evaluation research has focused on the long-term effects of an intervention. This study opens a unique window into the longevity of an intervention's outcomes and the factors that can help to sustain these. In this paper we report a long-term effect evaluation of a comprehensive intervention that focused on developing employeeship and that took place over six years in a University unit (the Intervention Unit, IU). A former evaluation showed that the intervention had been successful in achieving its outcomes in the first three years until 2015. In 2016, a major organizational merger was conducted. This provided a natural opportunity to evaluate the long-term sustainability of the intervention in terms of its effects on the psychosocial work environment. The present long-term effect evaluation is a multi-method study with contextual data from multiple sources and stakeholders. The final survey was administered one year after the merger. In addition to the survey, organizational records were examined and interviews with stakeholders were carried out to examine the contextual issues affecting the intervention. To evaluate the intervention, we used ARK, which is a systematic and validated tool. ANOVA analyses of the intervention-and control groups showed that the positive results were compromised in the follow-up survey (a further three years later). The contextual evaluation with ANCOVA pointed to main changes connected to the organizational merger and especially for the IU.