Abstract Context: It is unclear whether fat mass (FM) and lean mass (LM) differ in the way they influence cortical bone development in boys and girls. Objective: The aim of the study was to investigate the contributions of total body FM and LM to parameters related to cortical bone mass and geometry. Design/Setting: We conducted a longitudinal birth cohort study, the Avon Longitudinal Study of Parents and Children. Participants: A total of 4005 boys and girls (mean age, 15.5 yr) participated in the study. Outcome Measures: We measured cortical bone mass, cortical bone mineral content (BMCC), cortical bone mineral density, periosteal circumference (PC), and endosteal circumference by tibial peripheral quantitative computed tomography. Results: LM had a similar positive association with BMCC in boys and girls [regression coefficients with 95% confidence interval (CI); P for gender interactions: boys/girls, 0.952 (0.908, 0.997); P = 0.85]. However, the mechanisms by which LM influenced bone mass differed according to gender because LM was positively associated with PC more strongly in girls [boys, 0.579 (0.522, 0.635); girls, 0.799 (0.722, 0.875); P < 0.0001], but was only associated with cortical bone mineral density in boys [boys, 0.443 (0.382, 0.505); girls, 0.014 (−0.070, 0.097); P < 0.0001]. There was a stronger positive association between FM and BMCC in girls [boys, 0.227 (0.185, 0.269); girls, 0.355 (0.319, 0.392); P < 0.0001]. This reflected both a greater positive association of FM with PC in girls [boys, 0.213 (0.174, 0.253); girls, 0.312 (0.278, 0.347); P = 0.0002], and a stronger negative association with endosteal circumferencePC [boys, −0.059 (−0.096, 0.021); girls, −0.181 (−0.215, −0.146); P < 0.0001]. Conclusions: Whereas LM stimulates the accrual of cortical bone mass to a similar extent in boys and girls, FM is a stronger stimulus for accrual of cortical bone mass in girls, reflecting a greater tendency in females for FM to stimulate periosteal growth and suppress endosteal expansion.