Background: The research on job attainment and retention among young adults with serious mental illness (SMI) is limited. The objective of this study was to investigate the contributions of emotional, cognitive, motor, demographic, and work-related factors to the transition into supported employment (SE) and retention. Methods: This cross-sectional study included young adults with SMI involved in prevocational (N = 21) services or those who have transferred to SE (N = 21) following prevocational services. Work-related self-efficacy, executive functions, and motor skills were approached with standard and well-established tools. Results: There was a significant difference between groups in most dimensions of work-related self-efficacy, job history and experience, cognitive strategies, and general independence in daily life. The multivariate analysis demonstrates that holding a profession, experiencing self-efficacy in general work skills, cognitive strategies, and independence in living situations explained the between-group differences (χ2(4) = 34.62, p < 0.001; correct classification–90.2%). Conclusions: The study identifies the factors contributing to a sustainable transition to employment among young adults with SMI, suggesting the importance of a comprehensive approach to address a range of personal factors in an integrative way. The augmentation of prevocational training with continued employment support may be beneficial to meet the unique needs of young adults with SMI.