Objective Existing studies, in mostly male samples such as veterans and athletes, show a strong association between traumatic brain injury (TBI) and mental illness. Yet, while an understanding of mental health before pregnancy is critical for informing preconception and perinatal supports, there are no data on the prevalence of active mental illness before pregnancy in females with TBI. We examined the prevalence of active mental illness ≤2 years before pregnancy (1) in a population with TBI, and (2) in subgroups defined by sociodemographic, health, and injury-related characteristics, all compared to those without TBI. Method This population-based cross-sectional study was completed in Ontario, Canada, from 2012 to 2020. Modified Poisson regression generated adjusted prevalence ratios (aPRs) of active mental illness ≤2 years before pregnancy in 15,585 females with TBI versus 846,686 without TBI. We then used latent class analysis to identify subgroups with TBI according to sociodemographic, health, and injury-related characteristics and subsequently compared them to females without TBI on their outcome prevalence. Results Females with TBI had a higher prevalence of active mental illness ≤2 years before pregnancy than those without TBI (44.1% vs. 25.9%; aPR 1.46, 95% confidence interval, 1.43 to 1.49). There were 3 TBI subgroups, with Class 1 (low-income, past assault, recent TBI described as intentional and due to being struck by/against) having the highest outcome prevalence. Conclusions Females with TBI, and especially those with a recent intentional TBI, have a high prevalence of mental illness before pregnancy. They may benefit from mental health screening and support in the post-injury, preconception, and perinatal periods.