Abstract: Introduction: Although posttraumatic stress disorder (PTSD) and depression are prominent mental health conditions affecting United States service members, only a subset of individuals with these conditions utilize mental healthcare services. Identifying factors associated with mental healthcare utilization may elucidate military subgroups with unmet mental healthcare needs. Methods: Cross-sectional survey data from the 2019–2021 Millennium Cohort Study assessment were used to examine correlates of unmet mental healthcare needs among military personnel who screened positive for PTSD or depression symptoms (n=18,420) using modified Poisson regression models. Data analyses for this study were conducted between 2023 and 2024. Results: Approximately 32–43% of service members reported receiving any mental healthcare in the past 12 months. Hispanic and Asian or Pacific Islander personnel, and those with certain service characteristics (higher pay grade, recent deployment, experienced discrimination) had a lower likelihood of mental healthcare utilization. Female sex, greater symptom severity, experiencing bullying, and other psychosocial factors were associated with greater likelihood of mental healthcare utilization. Conclusions: One-third of service members with PTSD or depression symptoms reported any mental healthcare use, highlighting the need to identify factors that may impede or delay treatment. Racial and ethnic disparities in treatment utilization persist as do differences in utilization by military characteristics. Further research and initiatives are necessary to identify potential service-specific or cultural barriers and provide equitable quality and access to needed mental health services within the Military Health System.