--- - |2+ Abstract Background Developmental typologies regarding age of onset of violence and offending have not routinely taken account of the role of serious mental illness (SMI), and whether age of onset of offending in relation to onset of illness impacts on the manifestation of offending over the life course. Aims To test whether forensic psychiatric patients can be classified according to age of onset of SMI and offending, and, if so, whether subtypes differ by sex. Methods Details of all 511 patients enrolled into a large forensic mental health service in Ontario, Canada, in 2011 or 2012 were collected from records. Results A latent profile analysis supported a 2‐class solution in both men and women. External validation of the classes demonstrated that those with a younger age onset of serious mental illness and offending were characterised by higher levels of static risk factors and criminogenic need than those whose involvement in both mental health and criminal justice systems was delayed to later life. Conclusions Our findings present a new perspective on life course trajectories of offenders with SMI. While analyses identified just two distinct age‐of‐onset groups, in both the illness preceded the offending. The fact that our sample was entirely drawn from those hospitalised may have introduced a selection bias for those whose illness precedes offending, but findings underscore the complexity and level of need among those with a younger age of onset. Copyright © 2018 John Wiley & Sons, Ltd. - Criminal Behaviour and Mental Health, Volume 28, Issue 4, Page 335-349, August 2018.