This study identified longitudinal trajectories of lean mass development from childhood to young adulthood and examined their associations with bone and muscle outcomes in early adulthood. Data were obtained from 323 participants (8–23 years) in the Iowa Bone Development Study, assessed at 7 time points. Lean mass trajectories were defined using DXA-derived Z-scores for the appendicular lean mass index (aLBMI) and the appendicular lean mass to trunk fat mass ratio (aLBM/TrFM), categorized as low, average, or high. Only participants with consistent trajectories were included (n=116 for aLBMI; n=67 for aLBM/TrFM). At 23 years, bone material (DXA aBMD and pQCT vBMD), bone structure (pQCT geometry and strength), and muscle composition (pQCT muscle area and density) were assessed. Linear regression used the average trajectory as a reference and was adjusted for sex and height. A low aLBMI trajectory was associated with lower aBMD at the whole body less head (WBLH), femoral neck, and trochanter (−8% to −11%) and reduced tibial structural strength (−23% to −34%), with no differences at the radius. Higher aLBMI trajectories were associated with higher aBMD (up to +17%), greater bone strength in the tibia and radius (+22 to 45%), and larger muscle area (+32%). For aLBM/TrFM, only WBLH aBMD differed, with the high group showing ~5% higher values, along with lower BMI and higher muscle density. These findings suggest that consistent longitudinal patterns of lean mass development are associated with bone and muscle outcomes in early adulthood, with aLBMI more closely related to bone structure and muscle area, while a favorable muscle-to-fat ratio may reflect better muscle quality and whole-body bone status.