Objective: Despite the number of studies investigating co-occurring disorders, and more recently, co-occurring disorders and criminal offending, few studies have considered samples from forensic mental health services. The present study was conducted to investigate the relationship between mental illness, substance use disorders, antisocial personality disorder, and offending. Method: The prevalence of co-occurring disorders was investigated in 130 male offenders who had contact with the statewide forensic mental health service in Victoria, Australia. Offense histories and severity of offending were compared among participants diagnosed with a single mental illness (or no mental illness), co-occurring mental illness and substance use, and co-occurring disorders plus antisocial personality disorder. Results: The majority of participants had co-occurring mental and substance use disorders; a significant minority met the criteria for antisocial personality disorder. Participants with co-occurring mental illness and substance use disorders, and those who had an additional diagnosis of antisocial personality disorder, were responsible for more serious and frequent offending than those with mental illness alone. Conclusions and Implications for Practice: Forensic mental health services must take into account the effect that co-occurring disorders have on clients' functioning and offending. Those who work with people with psychiatric disabilities and co-occurring substance use disorders must ensure that the substance disorders are addressed to help ensure recovery from the mental illness and to reduce the likelihood of offending.