People experiencing mental illness smoke tobacco at disproportionately higher rates than the general population. Staff at mental health services are well placed to assist clients with smoking cessation, yet such support is often limited by time or knowledge constraints. Guided by the COM-B model, this study explored barriers and facilitators of smoking cessation among people receiving treatment for mental health disorders as perceived by staff members of community mental health services in Brisbane, Australia. Three focus groups were conducted in August-October 2021 with 29 healthcare professionals and peer support workers employed at three community mental health services. Data were analysed qualitatively using a combined deductive and inductive approach to identify themes grouped by the COM-B domains of capability, opportunity, and motivation. Capability barriers included clients' reliance on smoking to manage anxiety or other mental illness symptoms and difficulties engaging with cessation support, while staff knowledge was a facilitator. Opportunity barriers included smoking being perceived as social currency and clients’ limited support networks, whereas facilitators included cessation assistance from peer workers and smokefree environments. Motivation barriers were entrenched behaviours and using smoking as a coping mechanism, with facilitators including staff prioritising cessation and peer-led programs. These findings highlight the need for staff training to address misperceptions about smoking, improve confidence in offering cessation support, and foster trust with clients. Expanding the range of available smoking cessation support options, including peer support programs, and creating supportive service environments may enhance smoking cessation success for individuals experiencing mental illness.