Secondhand smoke is known to have adverse effects on the lung and vascular systems in both children and adults. It is unknown if childhood exposure to secondhand smoke is associated with adult mortality. The authors examined associations of childhood and adult secondhand smoke exposure with death from all causes, ischemic heart disease, stroke, and chronic obstructive pulmonary disease among 70,900 never smoking men and women, predominantly aged ≥50 years, from the Cancer Prevention Study–II Nutrition Cohort in 1992–1993. There were 25,899 participant deaths during follow-up through 2014. During 2016–2017, Cox proportional hazards regression models were used to calculate multivariable-adjusted hazard ratios and 95% CIs. Childhood secondhand smoke exposure was not associated with all-cause mortality. However, childhood secondhand smoke (living with a smoker for 16–18 years during childhood) was associated with higher mortality from chronic obstructive pulmonary disease (hazard ratio=1.31, 95% CI=1.05, 1.65). Adult secondhand smoke exposure of ≥10 hours/week at enrollment was associated with a higher risk of all-cause (hazard ratio=1.09, 95% CI=1.04, 1.14); ischemic heart disease (hazard ratio=1.27, 95% CI=1.14, 1.42); stroke (hazard ratio=1.23, 95% CI=1.04, 1.45); and chronic obstructive pulmonary disease (hazard ratio=1.42, 95% CI=0.97, 2.09) mortality. These results suggest that childhood secondhand smoke exposure, as well as adult secondhand smoke exposure, increase the risk of chronic obstructive pulmonary disease death in adulthood. Consistent with previous studies, the results also show that adult secondhand smoke is meaningfully associated with higher mortality from vascular disease and all causes. Overall, these findings provide further evidence for reducing secondhand smoke exposure throughout life.