ABSTRACT Introduction The impact of adverse childhood experiences (ACEs), warfare exposure, and mental health symptoms upon changes in body mass index (BMI) were examined in a large U.S. post-9/11 veteran sample to assess gender-specific changes in BMI within the first 2½ years after military service. Materials and Methods Data were collected with institutional approval in 6 waves between 2016 and 2019 from veterans who (1) separated from active duty component service branches (i.e., Army, Navy, Air Force, and Marine Corps) or National Guard or Reserve or (2) deactivated from active duty status after serving in a National Guard or Reserve component. Veterans self-reported height and weight at separation/deactivation at wave 2, and weight was asked at each subsequent wave. Multilevel growth model analyses estimated the relationship between ACEs, warfare exposure, and mental health symptoms and BMI for males and females. Weighted wave 5 analyses were conducted to ensure the sample was calibrated for nonresponse based on wave 1 and the full sample (n = 48,965) for each cross-classification of the weighting variables of gender, rank, and branch. Results Approximately one-third of the veterans reported a normal BMI at separation/deactivation in comparison to being overweight (51%) or obese (20%). Twenty-six percent of male veterans had a normal BMI in contrast to almost half of the female veterans. Male veterans who experienced 3-7 ACEs had a higher BMI (0.74) compared to male veterans without any ACEs, where a BMI increase of 0.08 per year was reported. Female veterans who experienced 1-2 ACEs had a higher BMI (0.89) compared to female veterans without ACEs. Male veterans who engaged in warfare (e.g., combat patrols and firing a weapon at enemy combatants) and experienced corollaries (i.e., consequences of combat) or who experienced corollaries alone (e.g., saw refugees who lost their homes/belongings) had higher BMIs (1.14 and 0.82, respectively) compared to male veterans without warfare exposure. Female veterans who experienced corollaries had a higher BMI (0.94) compared to female veterans with no warfare exposure. Female veterans who experienced warfare (i.e., corollaries and combat) had a higher BMI (0.71) compared to female veterans with no warfare exposure. Male veterans who screened positive for likely post-traumatic stress disorder (PTSD) or depressive symptoms had a higher BMI (1.01 and 0.52, respectively) compared to male veterans who did not screen positive. Male veterans who screened positive for likely PTSD increased their BMI by 0.10 per year. Male veterans who screened positive for both likely PTSD and depressive symptoms had a higher BMI (1.32) compared to male veterans who did not screen positive, and they increased their BMI by 0.21 per year. Female veterans who screened positive for likely PTSD and depressive symptoms had a higher BMI (0.78) and increased their BMI by 0.25 per year compared to female veterans who did not screen positive. Conclusions Boosting veterans’ and service members’ mental and emotional healing from childhood and warfare adversities through sound health promotion policies and increased access to evidence-informed interventions is imperative for optimal body weight and physical health.