Background: “Child maltreatment is a leading cause of health inequality” according to a leading WHO report. This statement is often assumed, yet, the size of the contribution of childhood adversity to the adult socioeconomic gradient in mortality remains unknown. Inequalities in mortality have mostly been investigated by taking adult conditions as a starting point. The objective of this study is to quantify how much of the socioeconomic gradient in adult life expectancy is associated with childhood adversity. Methods: Drawing on a 1953 birth cohort from Stockholm (n = 14 210), we compared inequalities in adult mortality within the full cohort to a counterfactual scenario where individuals with a history of childhood adversity (indicated by involvement with child welfare services) experienced the mortality rates of those achieving the same adult socioeconomic position, but with no history of childhood adversity. The socioeconomic gradient across education and income quintiles (attained by age 29) is measured by the slope index of inequality of temporary life expectancy (ages 29–67). Results: The counterfactual scenario attenuated the education gradient by 40 percent for men and 54 percent for women. Similarly, inequalities by income were reduced in the counterfactual scenario by 49 percent for men and 47 percent for women. Interpretation: These results support that childhood adversity is an important determinant of inequalities in mortality. The size of their contribution is equivalent to established behavioural risk factors. Taking a life course approach might provide important policy entry points to mitigate health inequalities.