Background: The arrival of one's first child is a known risk factor for mental illness, yet investigations on fathers' mental health are limited. We conducted a longitudinal investigation on paternal depression and anxiety in the nine years surrounding the transition to fatherhood. Methods: Using a national cohort of French men (CONSTANCES, n = 6299), we investigated the prevalence and associated risk factors of mental illness amongst first-time fathers. Responses to the Center for Epidemiological Studies Depression (CES-D) and 12-item General Health Questionnaire (GHQ-12) scales were used to identify clinically significant symptom scores. Self-declared mental illness was also reported by participants. Group-based modelling was used to identify latent trajectory groups for both measures. Results: Levels of self-declared anxiety (averaging 4.9 % pre-fatherhood, 7.8 % post) exceeded that of depression (1.9 % pre- fatherhood, 3.3 % post) or other disorders. However, rates of clinically significant symptom scores (17-27 %) were consistently higher. Participants' mental health appeared to worsen from two-years prior to their child's arrival and improve from two-years after. We identified three trajectory groups for fathers' self-declared mental illness: Low stable (90.3 %); Low risk with high temporary increase (5.6 %); and Consistent high risk (4.1 %). Risk factors associated with worsening mental health trajectories were unemployment, not living with one's partner, having had adverse childhood experiences and foregoing healthcare due to financial reasons. Limitations: All measures of mental illness relied on participant self-reports and are thus subject to bias. Conclusions: This study reveals an important period of heightened psychological vulnerability amongst first-time fathers, emphasising the need for increased and better adapted paternal mental health screening.