BACKGROUND: To further develop effective smoking cessation interventions within mental healthcare for people with severe mental illness (SMI), it is essential to gain insights into patients’ experiences with smoking (cessation), and professionals’ experiences with guiding patients in overcoming tobacco addiction. METHODS: We conducted 26 semi-structured interviews with 16 patients and 10 mental healthcare professionals (MHPs), as part of a one-year smoking cessation intervention. A purposive sampling strategy was applied to select the interviewees. All interviews were transcribed verbatim and thematically analysed using MAXQDA software. This study was embedded in a randomised controlled trial conducted in ambulatory mental healthcare in the Netherlands. RESULTS: Patients reported to smoke to cope with psychological distress and psychiatric symptoms, and to alleviate potential side effects of antipsychotic medication. For some patients low self-esteem and a lack of confidence in one’s own capacity to quit smoking were obstacles to a quit attempt. Therefore, for those patients these were crucial aspects to address. Patients and MHPs valued the exercises based on cognitive behavioural therapy (CBT). During group sessions, establishing personalised relapse prevention strategies was regarded as effective preparation for a quit attempt. The group setting was welcomed, however, adjustments to individual needs and preferences are required to personalise the intervention. CONCLUSIONS: Findings highlight the need for personalised care in treating tobacco addiction among people with severe mental illness. The KISMET intervention may serve as a useful framework for tailored cessation support, informed by the diverse experiences presented in this study. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13011-025-00663-9.