This study aimed to test the effectiveness of a nurse-led structured psychosocial intervention program in Chinese patients with first-onset mental illness. A single-blind, parallel group, randomized controlled trial design was used. The study involved 180 participants with mild to moderate-severe symptoms of psychotic or mood disorders who were newly referred to two psychiatric outpatient clinics in Hong Kong. Patients were randomly assigned to either an eight-session nurse-led psychosocial intervention program (plus usual care) or usual psychiatric outpatient care (both n=90). The primary outcome was psychiatric symptoms. Outcomes were measured at recruitment, one week and 12 months post-intervention. Patients in the psychosocial intervention group reported statistically significant improvements in symptoms compared to treatment as usual. There were also significant improvements in illness insight and perceived quality of life and reduction in length of re-hospitalizations over the 12-month follow-up. The findings provide evidence that the nurse-led psychosocial intervention program resulted in improved health outcomes in Chinese patients with first-onset mental illness.