Summer fruitlet thinning is implemented as a routine orchard practice to produce apple fruits with good quality. However, its impacts on the dynamics of fruit quality metrics during the growing season and in the postharvest storage, remain unclear. In this study, summer hand thinning on fruitlets of Ambrosia™ apple (Malus × domestica Borkh.) was conducted on two dwarfing rootstocks, Malling 9 (M.9) and Budagovsky (B.9), in an organic orchard and a conventional orchard under the semi-arid climate in Similkameen Valley, BC. Adequate thinning [(AT) in which 70% of fruitlets were removed] and light thinning [(LT) in which 30% of fruitlets were removed] were implemented in randomized plots in 8 wk after full bloom. Fruit development and dry matter content (DMC) were then monitored during the growing season; fruit quality was subsequently evaluated at harvest and after 4 mo of air storage at 0.5 °C. Relative to LT, AT enhanced fruit quality attributes in DMC, surface blush coverage and intensity, and soluble solids content (SSC) at harvest. The apples with higher DMC under AT also possessed higher compositional quality and lower incidences of fruit disorder in the postharvest stage. This study suggests that summer fruitlet thinning of Ambrosia apples can have significant impacts on fruit composition during subsequent on-tree fruit development, on the onset of ripening and eventually on the retention of quality and minimization of disorders over 4 mo of cold air storage. This effect is found for organic orchard (OG) and conventional orchard (CV) production systems and with both dwarfing rootstocks.