BACKGROUND: Mental health in the workplace is a growing concern for enterprises and policy makers. MENTUPP is a multi-level mental health intervention implemented in small and medium size enterprises from three work sectors in nine countries. This pilot study aimed to evaluate the feasibility, delivery, and instruments for the MENTUPP intervention to inform the planning of a clustered randomized controlled trial. METHODS: We administered items from the Copenhagen Psychosocial Questionnaire and the Danish Work Environment Cohort Study measuring psychosocial workplace factors. The questionnaire was answered by 382 participants at baseline, of which 98 participants also answered after six months at follow-up. We calculated mean scores of 19 psychosocial factors at baseline and conducted repeated measures ANOVAs to assess differences in eight psychosocial factors at follow-up. We also examined whether outcomes differed between work sectors and job positions at follow-up. RESULTS: The construction sector and workers with no or a lower leadership role reported more negative working environment factors at baseline. We observed a statistically significant decline in social support from colleagues and social community at work, and a marginally significant decline in justice at work. For the rest of the constructs, we did not observe statistically significant changes. CONCLUSIONS: We found significant differences in psychosocial work environment factors among work sectors and job positions at baseline. Contrary to our hypotheses, three psychosocial work environment factors decreased at follow-up. Possible explanations are the utilization of specific psychosocial factors as resources to cope with psychosocial stressors, high participant expectations that were not met by the intervention, insufficient time for structural changes, or the intervention prompting critical evaluations of the work environment. These findings will inform the design and implementation of the forthcoming clustered randomized controlled trial, where they will also be further investigated to validate their significance.