BACKGROUND: There is a growing awareness for addressing mental health in the workplace. Although interventions to promote mental health at the organizational-level exist, implementation is a challenge. Middle management can play a crucial role in implementing organisational-level interventions. Also, we know that mental models often need to change first, before enactment of such interventions occur. The aim of this study is therefore to better understand whether and how changes in mental models of middle managers are associated with the implementation activities of an organisational-level intervention to support mental health. Ultimately, this is meant to lead to the enactment of, in this case, a dialogue tool to normalize a conversation between middle manager and employee to enhance mental health. METHODS: Participatory Action Research was used as a method to design our implementation of a dialogue tool primarily focused on middle management to address mental in health in one SME company with 238 employees. In-depth interviews with 11 (middle) managers at the start of the implementation phase were held. After nine months, 9 out of the same 11 (middle) managers were interviewed again to gain understanding whether and how changes occurred during the implementation. Thematic analysis was applied to the interviews, from inductive and deductive perspective. Focus groups with employees and observations during the implementation activities were used for triangulation purposes. RESULTS: The mental models of the (middle) managers were analyzed based on readiness for change, perceived challenges and perceived opportunities. These mental models were generally positive towards the project, despite the low trust towards the Top Management and the general lack of experience with addressing mental health at the workplace. Nine months later, mental models changed towards more awareness and engagement in addressing mental health. Also, enactment of the dialogue tool by middle management and employees occurred. An association of these changes with, for example, the frequency and pace of implementation activities in which all employees were involved was reported. CONCLUSION: To implement interventions addressing mental health at the workplace, taking the time and using implementation activities that match the needs of middle management might help to change mental models of (middle) management.