Severe mental illness (SMI) and substance use disorders (SUD) are among the more chronic and costly mental health conditions treated in the Department of Veterans Affairs (VA). Service use patterns of returning veterans with SMI and SUD have received little attention. We examined gender differences in the utilization of VA services among a national sample of Operations Enduring Freedom, Iraqi Freedom, and New Dawn (OEF/OIF/OND) VA patients with SMI, SUD, and their comorbidity (SMI/SUD) in their first year of established VA care (N = 24,166). Outpatient services and acute–residential stays were modeled using negative binomial and logistic regression, respectively. Among all diagnostic categories, men used outpatient services less often than did women, including primary care (adjusted rate ratio [ARR] = .71, 95% confidence interval CI [.68, .74]), mental health (ARR = .85, 95% CI [.80, .91]), and addiction (ARR = .91, 95% CI [.83, .99]) services. For emergency department (ED) and psychiatric inpatient services, gender interacted significantly with diagnosis. The combination of SMI/SUD compared to either SMI or SUD conferred greater risk of ED utilization among men than women (adjusted odds ratio [AOR] = 2.09, 95% CI [1.24, 3.51], and 1.95, 95% CI [1.17, 3.26], respectively). SMI versus SUD conferred greater risk of psychiatric inpatient utilization among men than women (AOR = 1.83, 95% CI [1.43, 2.34]). Our findings point to gender differences in outpatient and acute service utilization among OEF/OIF/OND VA patients with some of the more chronic and costly mental health conditions. Further investigation of health care utilization patterns is needed to understand factors driving these gender differences to ensure that veterans have appropriate access to the services they need.