Background According to the Medical Research Council (MRC) framework, the theorisation of how multilevel, multicomponent interventions work and the understanding of their interaction with their implementation context are necessary to be able to evaluate them beyond their complexity. More research is needed to provide good examples following this approach in order to produce evidence-based information on implementation practices.

Objectives This article reports on the results of the process evaluation of a complex mental health intervention in small and medium enterprises (SMEs) tested through a pilot study. The overarching aim is to contribute to the evidence base related to the recruitment, engagement and implementation strategies of applied mental health interventions in the workplace.

Method The Mental Health Promotion and Intervention in Occupational Settings (MENTUPP) intervention was pilot tested in 25 SMEs in three work sectors and nine countries. The evaluation strategy of the pilot test relied on a mixed-methods approach combining qualitative and quantitative research methods. The process evaluation was inspired by the RE-AIM framework and the taxonomy of implementation outcomes suggested by Proctor and colleagues and focused on seven dimensions: reach, adoption, implementation, acceptability, appropriateness, feasibility and maintenance.

Results Factors facilitating implementation included the variety of the provided materials, the support provided by the research officers (ROs) and the existence of a structured plan for implementation, among others. Main barriers to implementation were the difficulty of talking about mental health, familiarisation with technology, difficulty in fitting the intervention into the daily routine and restrictions caused by COVID-19.

Conclusions The results will be used to optimise the MENTUPP intervention and the theoretical framework that we developed to evaluate the causal mechanisms underlying MENTUPP. Conducting this systematic and comprehensive process evaluation contributes to the enhancement of the evidence base related to mental health interventions in the workplace and it can be used as a guide to overcome their contextual complexity.