Abstract Background Early-onset anorexia nervosa (EO-AN) represents a significant clinical burden to paediatric and mental health services. The impact of EO-AN on bone mineral abnormalities has not been thoroughly investigated due to inadequate control for pubertal status. In this study, we investigated bone mineral abnormalities in girls with EO-AN regardless of pubertal development stage. Method We conducted a cross-sectional study of 67 girls with EO-AN (median age = 12.4 [10.9–13.7 years]) after a median duration of disease of 1.3 [0.6–2.0] years, and 67 healthy age-, sex-, pubertal status- matched control subjects. We compared relevant bone mineral parameters between groups: the total body bone mineral density [TB-BMD], the lumbar spine BMD [LS-BMD], the total body bone mineral content [TB-BMC] and the ratio of the TB-BMC to lean body mass [TB-BMC/LBM]. Results TB-BMD, TB-BMC, LS-BMD and TB-BMC/LBM were all significantly lower in patients with AN compared to controls. In the EO-AN group, older age, later pubertal stages and higher lean body mass were associated with higher TB-BMC, TB-BMD, and LS-BMD values. Discussion Girls with EO-AN displayed deficits in bone mineral content and density after adjustment for pubertal maturation. Age, higher pubertal stage and lean body mass were identified as determinants of bone maturation in the clinical population of patients with EO-AN. Bone health should be promoted in patients, specifically in those with an onset of disorder before 14 years old and with a delayed puberty.