Objective: To identify subgroups of forensic psychiatric patients based on the age onset of serious mental illness and offending and assess the external validity of the subgroups with theoretically based sociodemographic, clinical, legal and risk-related variables. Method: The age onset of serious mental illness and criminal contact was ascertained for a sample of 232 patients. A range of sociodemographic, clinical, legal and risk-related variables were coded to assess whether age onset subgroups differed in a manner consistent with the literature on typologies of mentally ill offenders. Results: One-quarter of the sample was classified as early starters (patients whose first offense occurred before becoming mentally ill), while two-thirds were late starters (where first offense occurred following illness onset). A small percentage (8%) of patients were deemed late late starters, defined as late starters who had experienced 10+ years of illness and were >37years upon first arrest. A larger proportion of early starters had a substance use disorder, antisocial personality disorder and a greater number of static/historical risk factors for violence. Early starters were younger upon first arrest and had more previous criminal contacts compared to late starters and late late starters. Mental illness was found to start later in life for late late starters; this group was also more likely to have been married and to have a spouse as victim in the index offense. Conclusion: We found support for distinct subgroups of mentally ill offenders based on the age onset of illness and criminal contact. Compared to late starters, offenses committed by early starters may be motivated more frequently by antisocial lifestyle and attitudes, as well as more instrumental behaviors related to substance abuse. In addition, late late starters may represent a distinct third subgroup within late starters, characterized by relatively higher levels of functioning and social stability; future work should replicate. Findings suggest different rehabilitation needs of the subgroups.