Abstract Objective Mothers of children born very preterm (VP) are at increased risk of developing postnatal depression, anxiety, and post-traumatic stress symptoms. However, mental health disorder rates are rarely assessed in this population compared with full-term peers, and it is unclear if postnatal distress symptoms precede higher rates of maternal mental health disorders at 5 years post-birth in both birth groups. Methods Mothers of children born VP (n = 65; mean [SD] age at birth, 33.9 [5.0]; 72.1% tertiary educated) and full-term (n = 90; mean [SD] age at birth, 33.4 [4.0]; 88.2% tertiary educated) completed questionnaires assessing symptoms of depression, anxiety, and trauma within 4 weeks of birth. At 5 years post-birth, they participated in a structured diagnostic interview assessing mood, anxiety, and trauma-related mental health disorders, both current and over the lifetime. Results There was little evidence for differences between mothers in the VP and full-term groups in rates of any mental health disorder at 5 years (VP = 14%, full-term = 14%) or lifetime (VP = 41%, full-term = 37%). In mothers of children born VP, elevated postnatal post-traumatic stress symptoms were associated with higher rates of mental health disorders at 5 years (odds ratio = 21.5, 95% confidence interval = 1.35–342). Conclusions Findings suggest that preterm birth may not lead to increased odds of later developing maternal mental health disorders, despite known risks of elevated postnatal distress following a VP birth. However, those with post-traumatic stress symptoms following a VP birth could be more vulnerable, and assessment and monitoring is recommended.