The majority of all treated cases of traumatic brain injury are classified as being in the mild severity range (MTBI) but many symptoms are far from mild and can result in difficulties that persist for years after the initial injury, severely affecting quality of life. Chronic depression and anxiety symptoms can be masked by acute post-concussion symptoms and can affect males and females differently in the long-term but little research has been undertaken in this area. The aims of this study were to investigate the gender differences in depression and anxiety over 1-month, 12-months and 8-years following a mild traumatic brain injury and to investigate gender differences in depression and anxiety between a group of MTBI subjects (8-years post-injury), and a group of participants who were TBI-free, by comparing Hospital Anxiety and Depression Scale (HADS) scores and self-reported symptoms. A MTBI cohort consisting of a population-based sample of 151 adults who sustained a mild TBI between 2010 and 2011 was identified from an earlier longitudinal study: Brain Injury Incidence and Outcomes New Zealand In the Community Study (BIONIC). Additionally, 213 participants with no history of head injury were recruited for comparison. Both groups answered questions about current anxiety and depression symptoms using the HADS. Overall, results suggest that males and females with MTBI have the same overall course of recovery in relation to depression and anxiety symptoms over the 8-years following injury but that females consistently report significantly increased symptoms than males over time, as reflected in the general population. Depression and anxiety scores fluctuate over the 8-years post-MTBI but, overall, depression increases, anxiety decreases and symptom levels are below clinical severity. Females with MTBI are significantly more likely than males to experience anxiety in the clinical range at 12-months post-injury. There were no significant gender differences in depression or anxiety levels between the MTBI and comparison groups, and average HADS scores were not significantly different between groups. However, individual symptom analysis showed that females who were 8-years post-MTBI experienced increased panic symptoms than females with no TBI and that both males and females with MTBI reported increased anhedonia and “restlessness’ symptoms than those without MTBI. Overall, females reported significantly higher severity symptoms related to nervousness and feeling slowed down than males. The clinical implications of this study suggest a focus on the longer-term impacts of MTBI as patients may experience later onset depression or anxiety that may not be present in the acute phase. Evaluating individual symptoms as well as overall scores on measures can identify important clinical effects at symptom level that are masked by total score analysis. Practitioners should consider gender differences in how patients respond to self-report symptom measures to ensure accurate clinical assessment of both males and females, particularly in relation to MTBI which has a higher male prevalence. Future research into MTBI outcomes that includes a broader range of measures, in addition to self-report, will ensure comprehensive clinical data is obtained about individual symptoms and risk factors. This study uniquely contributes to current research into gender outcomes post-MTBI and may guide clinical practice to develop effective, timely and targeted assessment and treatment interventions for depression and anxiety symptoms experienced by both males and females after mild traumatic brain injury.