Mental health data from the 2016-2017 Vietnam Era Health Retrospective Observational Study (VE-HEROeS) were analyzed by cohort, represented by United States Vietnam theater veterans (VTs) who served in Vietnam, Cambodia, and Laos; nontheater veterans (NTs) without theater service; and age- and sex-matched nonveterans (NVs) without military service. The exposure of interest was Vietnam theater service. Surveys mailed to random samples of veterans (n = 42,393) and nonveterans (n = 6,885) resulted in response rates of 45.0% for veterans (n = 6,735 VTs, M = 70.09, SE = 0.04; n = 12,131 NTs) and 67.0% for NVs (n = 4,530). We examined self-report data on four mental health outcomes: probable posttraumatic stress disorder (PTSD), depression, psychological distress, and overall mental health functioning. Weighted adjusted odds ratios (aORs) between each outcome and cohort were estimated, controlling for covariates in four models: cohort plus sociodemographic variables (Model 1), Model 1 plus physical health variables (Model 2), Model 2 plus potentially traumatic events (PTEs; Model 3), and Model 3 plus other military service variables (Model 4). Mental health outcome prevalence was highest for VTs versus other cohorts, with the largest aOR, 2.88, for PTSD, 95% CI [2.46, 3.37], p < .001 (Model 4, VT:NT). Physical health and PTEs contributed most to observed effects; other service variables contributed least to aORs overall. Mental health dysfunction persists among VTs years after the war's end. The present results reaffirm previous findings and highlight the need for continued mental health surveillance in VTs.