Socioeconomic inequalities in body weight have been demonstrated in numerous cross-sectional studies; however, little research has investigated these inequalities from a life course and longitudinal perspective. We examined the association between child- and adulthood socioeconomic position (SEP) and BMI and overweight/obesity in 1991 (baseline) and changes in BMI and the prevalence of overweight and obesity between 1991 and 2004. Data from the 1991 and 2004 waves of the longitudinal Dutch GLOBE study were used. Participants (n = 1,465) were aged 40–60 years at baseline. BMI was calculated from self-reported height and weight collected by postal questionnaire. Retrospective recall of father's occupation was used as childhood socioeconomic indicator, and adulthood SEP was measured by the occupation of the main income earner of the household. The findings showed that among women, childhood SEP exerted a greater influence on body weight than SEP in adulthood: at baseline, women from disadvantaged backgrounds in childhood had a higher BMI and were more likely to be overweight or obese, and they gained significantly more weight between baseline and follow-up. In contrast, adult SEP had a greater impact than childhood circumstances on men's body weight: those from disadvantaged households had a higher mean BMI and were more likely to be overweight or obese at baseline, and they gained significantly more weight between 1991 and 2004. The findings suggest that exposure to disadvantaged circumstances at critically important periods of the life course is associated with body weight and weight gain in adulthood. Importantly, these etiologically relevant periods differ for men and women, suggesting gender-specific pathways to socioeconomic inequalities in body weight in adulthood.