To date, HCI for mental health has primarily responded to challenges in the treatment of mental illness, with a focus on therapy access and engagement. However, approaches to improving and protecting people’s mental wellbeing have received less attention. Prompted by recent discussions in Western Healthcare and Psychology, we argue for a more holistic approach to promoting mental health that expands the field’s focus to include strategies for enhancing mental wellbeing. A closer consideration of mental wellbeing can increase the effectiveness of mental health interventions, help in preventing mental illness and relapse, and extend our knowledge as to how we can support people to flourish as individuals and enhance their quality of life more generally. Our aim is to encourage more research on positive aspects of mental health in the treatment and care provision of people with mental health problems, and to support preventive approaches. To this end, the paper provides a comprehensive definition of mental wellbeing as positive emotional, psychological and social health; presents a review of HCI literature illustrating how the field is beginning to respond to the mental wellbeing agenda; and proposes avenues for future design and research in this area.