Weight gain and obesity has reached epidemic proportions, with the prevalence of metabolic syndrome (MetS) reaching 20–25% of the global population. MetS is a cluster of metabolic abnormalities, including weight gain, associated with an increased risk of cardiovascular disease, diabetes, and stroke. While individuals in the general population are at risk of physical conditions, such as MetS, people with mental illness are at even higher risk. The increased incidence of MetS for people with serious mental illness has been linked to the use of second-generation antipsychotic medication. This paper describes the content, structure, and development of ‘Passport 4 Life’, a healthy lifestyle intervention to maintain weight and prevent further weight gain for people with serious mental illness. While there are a number of diet and lifestyle programmes available for the general population, Passport 4 Life was specifically designed as an intervention to address the needs of people with serious mental illness taking second-generation antipsychotics. Passport 4 Life comprises 12 weekly group sessions that include the concepts and spirit of motivational interviewing, nutrition, and exercise education, combined with weekly exercise activity.