Twin pregnancy is a risk factor for postpartum depression and anxiety. Whether this translates into a higher risk of severe maternal mental illness in the short-term or long-term is unknown. This study was a population-based retrospective cohort study, using linked health administrative databases for the entire province of Ontario, Canada. Included were primiparas aged 15–50 years with a twin vs. singleton hospital livebirth, between January 1, 2003, and March 31, 2019. Propensity-score inverse probability of treatment weights accounted for potential confounding. The primary outcome of severe mental illness comprised a composite of an emergency department visit or hospitalization for mental illness or self-injury, or death by suicide, assessed in the first year after birth, and in long-term follow-up, up to 17 years thereafter. Fifteen thousand twenty-four twin and 796,804 (15,022 weighted) singleton births were included, with a mean (IQR) duration of follow-up of 9 (5–13) years. After weighting, the mean (SD) maternal age was 31.3 (5.5) years. In the first 365 days postpartum, severe mental illness occurred at rates of 10.5 and 8.7 per 1000 person-years in twin and singleton mothers, respectively, corresponding to a hazard ratio (HR) of 1.21 (95% CI 1.07–1.47). From 366 days onward, the corresponding figures were 5.9 and 6.1 per 1000 person-years (HR 0.96, 95% CI 0.89–1.04). Individuals with a twin birth appear to experience an increased risk for severe mental illness in the first year postpartum, but not thereafter. This suggests a potential need for targeted counselling and mental health services for mothers within the first year after birth.