Diagnosis of mental disorder is prevalent among people who have been incarcerated. Nevertheless, community mental health services are often limited following release from prison, and reincarceration rates are high. The prevalence of mental disorders is growing among people who are incarcerated in British Columbia (BC), Canada, increasing the urgency of timely and accessible mental health services after release. To examine the association of mental health services access and timeliness of services access with reincarceration risk among people released from prison. In this cohort study, mental disorder diagnoses were derived from International Classification of Diseases, Ninth Revision or International Statistical Classification of Diseases and Related Health Problems, Tenth Revision codes in administrative health records. Data on prison release and reincarceration were retrieved from corrections records. Population-based health and corrections data were retrieved from the BC Provincial Overdose Cohort, which contains a 20% general population random sample of 1 089 677 BC residents. This study examined releases from provincial prisons between January 1, 2015, and December 31, 2018, among people in the 20% random sample who had a mental disorder diagnosis in the year before their release. Analyses were performed from January to June 2022. Mental health services access (primary care, emergency department visits, or hospitalization) and sociodemographic, health, and incarceration characteristics. A multistate modeling approach was taken. Cox proportional hazards models were stratified by transition, from release to reincarceration, with and without mental health services access. A state arrival extended model examined the influence of timeliness of mental health services access on subsequent hazard of reincarceration. A total of 4171 releases among 1664 people (3565 releases among male individuals [84.6%]; 2948 releases [70.7%] among people <40 years old; 2939 releases [70.5%] among people with concurrent substance use disorder diagnosis) were identified. The total study follow-up time was 2834.53 person-years, with a mean (SD) of 0.68 (0.93) years and median (IQR) of 0.25 (0.07-0.84) years per release. Mental health services access was associated with a reduction in the hazard of reincarceration (hazard ratio, 0.61; 95% CI, 0.39-0.94). For each additional month between release and mental health services access, the hazard of reincarceration was increased by 4% (hazard ratio, 1.04; 95% CI, 1.01-1.07). In this cohort study of people with mental disorder diagnoses released from prison in BC, mental health services access was associated with reduced reincarceration risk. These findings suggest that these services may have the greatest impact on reducing reincarceration risk when they are available in a timely manner in the days and weeks immediately following release.