Purpose: Studies examining mental disorders among women have primarily focused on either depression, anxiety, or substance use disorders and not included the broader spectrum of mental disorders. Mixed evidence exists on the prevalence rates of mental disorders among mothers. This study compares the prevalence of different mental disorders and mental comorbidities between mothers and non-mothers and assesses correlates of mental disorders among mothers. Methods: A population-based birth cohort design was adopted, consisting of 40,416 females born in Queensland, Australia, in 1983/84. Linked administrative data from hospital admissions were used to identify mental disorders. Cumulative incidence curves of different mental disorders were created separately for mothers and non-mothers. Results: Mental disorder prevalence among females by age 29–31 years was 7.8% (11.0% for mothers and 5.2% for non-mothers). Mothers were overrepresented in almost all categories of mental disorders, with overrepresentation becoming more pronounced with age. Mothers with a mental disorder were more likely to be unmarried, Indigenous, young at birth of first child, have greater disadvantage, and have a single child, compared to mothers without a mental disorder. Nearly half of the mothers (46.9%) had received a mental disorder diagnosis before having their first child. Conclusions: Mothers, particularly unmarried, Indigenous, having greater disadvantage, and younger at birth of first child, represent a unique group with high vulnerability to mental disorders, that begins in childhood and is amplified with age. Presence of significant mental disorder comorbidities among females highlights the critical importance of a comprehensive, integrated approach to prevent and address multiple comorbidities.