Abstract: Introduction: Adolescent mental health and family functioning have received limited attention in UK military families. This study investigated their association with posttraumatic stress disorder (PTSD) and depression in military fathers. Methods: In total, 105 serving and ex-serving members of the UK Armed Forces, and 137 of their adolescent offspring (ages 11 to 17 years), were included in this cross-sectional secondary data analysis. Data were collected online and at home using validated questionnaires. Results: Probable PTSD or depression was associated with more impaired general family functioning (unadjusted b = 0.21; 95% CI, 0.07–0.35; p = 0.003) and increased likelihood of adolescent mental health disorders (unadjusted OR = 2.30; 95% CI, 1.10–4.81; p = 0.027). The direction and strength of these associations did not substantially change after adjusting for covariates. Discussion: This highlights the importance of supporting the well-being of military families, especially when parents have mental health problems.