It is unclear if body weight in early life affects cancer risk independently of adult body weight. To investigate this question for six obesity-related cancers, we performed univariable and multivariable analyses using i) Mendelian randomization (MR) analysis and ii) longitudinal analyses in prospective cohorts. Both the MR and longitudinal analyses indicated that larger body size at age 10 was associated with higher risk of endometrial (ORMR=1.61, 95%CI = 1.23-2.11) and kidney cancer (ORMR=1.40, 95%CI = 1.09-1.80). These associations were attenuated after accounting for adult body size in both the MR and cohort analyses. Early life BMI was not consistently associated with the other investigated cancers. The lack of clear independent risk associations suggests that early life BMI influences endometrial and kidney cancer risk mainly through pathways that are common with adult BMI.