Background: Compared to the general population, people who are at a high risk of or experience severe mental illness (SMI) such as psychosis, are more likely to have low levels of physical activity, high levels of sedentary behaviour, and sleep problems. Intervention: The Well-Track intervention comprises a wearable activity and sleep tracker (Fitbit); one session with mental health service staff providing physical activity and sleep hygiene advice; a brief motivational interview; completing a goal setting workbook; and one or two further engagement, feedback and discussion sessions. Participants: Twenty-four participants using an early intervention in psychosis (EIP) or at-risk mental state (ARMS) service completed an eight-week, three session intervention (14 males and 10 females), with an age range of 18 -61, and average age of 27.75 years. Methods: An open-label patient cohort design with no control group. Pre-intervention, 4-week and 8-week intervention assessments using participant self-report measures: Patient Health Questionnaire (PHQ-9) (depression), Warwick-Edinburgh Mental Wellbeing Scale (WEMWBS), and Sleep Conditioning Index (SCI); and clinician measurement of body weight. Results: Mean scores showed a significant improvement in PHQ-9 from 9.29 (SD 5.89) to 5.58 (SD 3.68) at 4 weeks and to 5.83 (SD 4.40) at 8 weeks, with large effect sizes. For those who met a diagnosis of depression at baseline, at 4 week follow-up seven participants (26%) experienced remission and nine (33%) reliable improvement, and at 8 week follow-up four (21%) experienced remission and seven (37%) reliable improvement.