The effects of harvest date (5 dates, between 100 and 140 days after full bloom) and carbon supply (2 leaf-to-fruit ratios, 10 and 100) on mango fruit (cv. Cogshall) quality and maturity were studied to find reliable indicators that take fruit physiological maturity into account and to establish a compromised harvest date according to the market. Fruit size and density varied with regard to the harvest date and assimilate supply. Changes in total soluble solids and titratable acidity were well correlated with concentrations of major soluble sugars and organic acids. Potassium concentration was increased according to harvest date. Metabolic changes occurred at 126–133 and 133–140 days after full bloom in fruit subjected to non-limited and stress conditions of assimilate supply, respectively. During this stage, sucrose and malic acid concentrations strongly increased, whereas those of starch and citric acid decreased according to the leaf-to-fruit ratio treatment. Synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC) and malonyl ACC, and a rapid increase in the respiration rate were observed during this period. The shortage of assimilate supply delayed the onset of maturation and reduced its intensity. Storage quality of mango cv. Cogshall may be influenced by assimilate supply since the K⁺ to Ca²⁺ ratio and the pulp dry matter content, in particular, were enhanced in fruit from the 100 leaf-to-fruit ratio treatment. Results for mango cv. Cogshall suggested that total soluble solids, sucrose, and malic/citric acid ratio can be used as physiological indices for mango fruit, whereas fruit fresh mass, density, and pulp dry matter content can be useful indicators for when to harvest fruit. It appeared that 133 days after full bloom was an optimal harvest date for fruit grown under non-limited conditions of assimilate supply, for a market where the time between harvest and consumption of fruit is short.