Purpose: To examine the impact of substance use and other risk factors on conviction rates in people with a psychotic illness (PI) and other mental disorders (OMD) compared to those with no mental illness (NMI). Methods: This research is part of a longitudinal record-linked whole-population study of 467,945 children born in Western Australia (WA) between 1980 and 2001. This cohort was identified through linkages between the WA psychiatric case register, WA corrective services data and other state-wide registers. We assessed 184,147 individuals born during 1983–1991 to explore the impact of exposure to a variety of risk factors on conviction rates. Results: People with PI and OMD had higher conviction rates than those with NMI, with unadjusted incidence rate ratios (IRR) of 3.98 (95% CI 3.67–4.32) for PI and 3.18 (95% CI 3.03–3.34) for OMD. Adjusting for substance use reduced the rates by 60% in PI and 30% in OMD: IRRs 1.59 (95% CI 1.45–1.74) and 2.24 (2.12–2.37), respectively. Minimal change was seen when adjusting for other potential risk factors (including socio-demographics, victimisation and parental offending), with adjusted IRRs 1.58 (95% CI 1.43–1.74) for PI and 1.90 (95% CI 1.80–2.02) for OMD. Conclusions: Our analysis shows people with a mental illness have higher rates of conviction than those with NMI. Substance use has a major impact on this rate. Results suggest the need for a greater investment in programs addressing the issue of comorbid substance use with a view to reduce the rate of convictions in this population.