Young children's symbolic magnitude understanding, or knowledge of how written numerals and number words can be ordered and compared, is thought to play an important role in their mathematical development. There is consistent evidence that symbolic magnitude skills predict mathematical achievement in later childhood and adulthood. Yet less is known about symbolic magnitude understanding before the start of formal schooling, a time when children are rapidly developing knowledge of small whole numbers. In this study, preschoolers (N = 140, Mean age = 4 years, 5 months) were assessed using measures of numerical skills (cardinality, symbolic magnitude, addition) and executive functioning (working memory, inhibitory control, attention shifting) in the winter and spring of the school year. Symbolic magnitude predicted later addition skills, fully mediating the relation between children's cardinality and addition skills. Moreover, children's domain-general executive functioning skills and domain-specific numeracy skills explained a similar amount of variability in children's later addition skills. Results highlight the role of symbolic magnitude in the development of children's understanding of mathematics.