The purpose of this study is to determine the validity of the work engagement construct among mentally ill workers and to develop a nomological network delineating the relationship of work engagement with its antecedents, and its consequences in this specific population. Three hundred and ten people with mental disorders employed in Italian social enterprises accepted to take part in this longitudinal study and filled out the Utrecht Work Engagement scale (UWES-9) and questionnaires on severity of symptoms perceived, social support from coworkers and supervisor, and occupational self-efficacy. Individuals who were still eligible at the 12-month follow-up phase of the study completed a questionnaire on future working plans. Confirmatory factor analysis and path analysis were used to validate the UWES-9 and test its nomological network. The results indicated how work engagement, as well as its drivers, impacted important work outcomes, such as to work in the open labor market, in workers with mental disorders.