Purpose: The purpose of this study was to examine the direct and indirect relations between short-term memory (STM), reading, oral language, and writing at the letter, word, and discourse levels in young, developing writers both cross-sectionally and longitudinally. Method: Participants were 449 English-speaking kindergarten students (52% female) from 41 schools in the U.S. Results: The cross-sectional analysis indicated that STM was directly related to oral language ([beta] = 0.62), letter naming fluency ([beta] = 0.43), word reading ([beta] = 0.54), and reading comprehension ([beta] = 0.42), all of which were statistically significant. Results supported a complete mediation model with STM having an indirect effect on writing via reading and oral language. In the cross-sectional analysis, the total effects of STM on handwriting ([beta] = 0.51), spelling ([beta] = 0.57), and written composition ([beta] = 0.47) were substantial and statistically significant. In the longitudinal analysis, the total effects of Fall STM on Spring writing outcomes, handwriting ([beta] = 0.42), spelling ([beta] = 0.70), and written composition ([beta] = 0.44) were also substantial and statistically significant. Conclusion: Findings confirm that STM is a foundational cognitive skill necessary for early writing; however, they provide a more nuanced understanding by showing that the contribution of STM is indirect through reading and oral language. Writing is complex, requiring interaction among higher-order domain-specific and domain-general components. Along with other recent studies, our findings highlight the need to examine the direct and indirect effects for a more precise understanding of skills that support writing development.