The positive correlation of mortality risk with early and mid-life BMI is well-documented for modern populations but less well known for earlier cohorts when average BMI was lower and medical knowledge more limited. Recent scholarship has called into question the assumption that the mortality risk of being overweight is unchanged historically. We use two large samples of New Zealand men who survived World War I or World War II, and trace nearly 80% to death records. We find no evidence of higher mortality risk for men with low BMI, but in both cohorts there was a significantly greater risk of early mortality for men with a BMI over 27 at enlistment. However, the magnitude of the increased risk from being overweight halved between the two cohorts. Thus our study contributes new evidence that twentieth century improvements in nutrition and medicine allowed people to live longer at high body masses.