Childhood obesity rates have reached epidemic levels with Mediterranean countries reporting among the highest numbers globally. Evidence suggests early life factors, including infant growth rate, increase the risk of obesity later in childhood. However, optimal rates of infant growth associated with lower future obesity risk remains undetermined. The study aim was to determine the optimal infant growth rate associated with a lower risk of childhood overweight and obesity. Perinatal and anthropometric data collected from 1,778 Greek preschool (2–5 years old) and 2,294 Greek preadolescent (10–12 years old) children participating in the ToyBox and the Healthy Growth Study (HGS) respectively, was combined for examination. Logistic regression models and receiver operating curves were used to determine the association between infant growth rate and development of childhood overweight and obesity, as well as optimal infant growth rate, respectively. Rapid weight gain during the first 6 months of life was positively associated with overweight and obesity in preadolescent children (OR:1.36, 95% CI: 1.13–1.63). Optimal cut-off points for several infancy growth rate indices (i.e., WAZ, WLZ, HAZ, BAZ) associated with a lower risk of overweight and obesity in preschool years and preadolescence were also identified. The current findings have the potential to guide health professionals in monitoring infant growth to prevent childhood overweight and obesity, through timely identification of those at high risk and effective early intervention.