Background: Mental health conditions prior to or during pregnancy that are not addressed can have adverse consequences for pregnancy and birth outcomes. This study aimed to determine the extent to which women's mental health-related hospitalisation (MHrH) prior to or during pregnancy was associated with a risk of adverse birth outcomes. Methods: We linked the perinatal data register for all births in the Northern Territory, Australia, from the year 1999 to 2017, to hospital admissions records to create a cohort of births to women aged 15–44 years with and without MHrH prior to or during pregnancy. We used Modified Poisson Regression and Latent Class Analysis to assess the association between maternal MHrH and adverse birth outcomes (i.e., stillbirth, preterm birth, low birth weight, and short birth length). We explored a mediation effect of covariates on theoretical causal paths. We calculated the adjusted Population Attributable Fraction (PAF) and Preventive Fractions for the P