Early growth pattern is increasingly recognized as a determinant of later obesity. This study aimed to identify the association between weight gain in early life and anthropometry, adiposity, leptin, and fasting insulin levels in adolescence. A cross-sectional study was conducted in 366 school children aged 11–13 years. Weight, height, and waist circumference (WC) were measured. Fat mass (FM) was assessed using bioelectrical impedance analysis. Blood was drawn after a 12-h fast for insulin and leptin assay. Birth weight and weight at 6 months and at 18 months were extracted from Child Health Development Records. An increase in weight SD score (SDS) by ≥0.67 was defined as accelerated weight gain. Linear mixed-effects modeling was used to predict anthropometry, adiposity, and metabolic outcomes using sex, pubertal status, accelerated weight gain as fixed factors; age, birth weight, and family income as fixed covariates, and school as a random factor. Children with accelerated weight gain between birth and 18 months had significantly higher body mass index (BMI) SDS, WC SDS, height SDS, %FM, fat mass index (FMI), fat free mass index (FFMI), and serum leptin levels in adolescence. Accelerated weight gain between 6 and 18 months was associated with higher BMI SDS, WC SDS, %FM, and FMI, but not with height SDS or FFMI. Accelerated weight gain at 0–6 months, in children with low birth weight, was associated with higher height SDS, BMI SDS, WC SDS, %FM, and FMI; in children with normal birth weight, it was associated with BMI SDS, WC SDS, height SDS, and FFMI, but not with %FM or FMI. Effects of accelerated weight gain in early life on anthropometry and adiposity in adolescence varied in different growth windows. Accelerated weight gain during 6–18 months was associated with higher FM rather than linear growth. Effects of accelerated weight gain between 0 and 6 months varied with birth weight.