Abstract Background People experiencing mental illness (PMI) face significant challenges in accessing and using somatic healthcare services, reducing their overall life expectancy by up to 20 years. The COMITMENT project aimed to identify barriers and facilitators in accessing and using somatic healthcare services for PMI and co-design strategies that could address existing challenges. Methods This study employed a qualitative participatory approach, involving a “trialogic” steering group composed of a person with lived experience of mental illness, a family member and caregiver of PMI, as well as a health professional. Two co-design workshops were conducted with members of each trialogic group, based on “World Café” methodology, facilitating open discussion between and among the three perspectives. Data was gathered through participant observations and systematically collecting participants’ workshop contributions, and analyzed thematically. Results Overall, 52 people participated in the two workshops. Findings identified six overarching themes regarding challenges in somatic healthcare access/use for PMI: (1) accessibility and facility design, (2) support and bureaucracy, (3) awareness among healthcare professionals, (4) societal awareness, (5) structural diversity of services, and (6) lack of networking abilities. Notable insights include inclusivity design requirements for healthcare facilities and the importance of integrating lived experiences of PMI into the training of healthcare professionals to reduce stigma. Conclusions This study underscores the critical need for systemic changes to improve somatic care for PMI. It suggests that greater awareness among healthcare professionals, coupled with more inclusive service design and enhanced networking abilities, could substantially improve the quality of healthcare for PMI. The COMITMENT project provides robust and co-designed results for social, systemic, and political reforms aimed at reducing healthcare inequity. Key messages • This study shows how direct involvement of relevant stakeholders improves the design of interventions that truly meet the needs of people with mental illness regarding somatic healthcare services. • To improve healthcare access and treatment for people experiencing mental illness, systemic social, design, educational, and political changes and adaptations are required.