Abstract Background Mounting public health threats associated with climate change, epidemiological and economic crises, digitalisation, rising migration, and population aging have uncovered at-risk groups’ increased vulnerability to mental ill health. Developing mental health promotion interventions tailored to their specific needs and expectations is essential to protect their wellbeing. However, researchers may find it challenging to identify and implement needed adaptations without engaging with end-users, and other stakeholders, to co-create tailor-made interventions. This study aimed to explore co-creators’ views about opportunities and constraints in participatory mental health promotion research. Methods Between September 2023 and July 2024, a qualitative study drawing on participant observation of bimonthly meetings and semi-structured interviews with six co-creation facilitators was conducted in the scope of Horizon Europe’s project ADVANCE. The multi-methods research approach to co-creation entailed desk reviews and online Delphi surveys co-developed with locally-established Society Advisory Groups, and scenario-based workshops held with stakeholders in the six partner countries: Denmark, Germany, Italy, Lithuania, Netherlands, Switzerland. Thematic analysis of the data was carried out. Results Research teams’ epistemic stances impact research planning, resource allocation, partnership nourishing and the unravelling of co-option risks during participatory research implementation. Research co-creation adds pressure onto research teams’ agenda, skillset and performance, but it also encourages power sharing, mutual learning and greater alignment of research design and implementation with end-users’ priorities and aspirations. Conclusions Research co-creation is a partnership-based and resource-intensive endeavour that can provide a valuable pathway to improve the relevance, acceptability and sustainability of public mental health promotion research. Key messages • Research co-creation demands planning, adequate resource allocation, flexibility and continuous investment to sustain academy-society partnerships. • Engaging stakeholders in knowledge co-creation can increase the relevance, acceptability and sustainability of public mental health promotion research.