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Magnesium (Mg) is essential to many plant physiological and biochemical processes; however, understanding how Mg nutrition quantitatively affects the production, partitioning, and utilization of photoassimilates is still lacking, especially in soilless culture systems. We focused on the roles of Mg in yield formation and interactions with potassium (K) nutrition of cherry tomato. Cherry tomato yield, photosynthetic parameters, dry matter weight, and K, Mg, and calcium (Ca) uptake were investigated in two soilless experiments with seven Mg levels and five K levels. The results showed that low (4 mM) Mg supply limited cherry tomato yield by decreasing dry matter accumulation by 22.6–78.1% and harvest index by 13.9–40.7%. The critical leaf Mg concentrations required for adequate photosynthate production in the first and second harvest periods were 4.67 and 5.52 g·kg−1, respectively. However, over-supply of Mg reduced leaf K and Ca concentrations and limited plant uptake of K and Ca. Moreover, adjusting K concentrations in solution could influence plant Mg functions in photosynthesis and, therefore, cherry tomato growth. Overall, balanced Mg and K application increased Mg, K, and Ca uptake, as well as Mg concentrations in leaves, which could maintain a sustainable photosynthetic rate and plant dry matter formation.