Cross-sectional studies have already addressed the question of the syllable's role in the first steps of reading acquisition--though with mixed results. To determine whether and when (1) syllables become units that drive the segmentation of and access to words and how (2) sublexical orthographic and phonological syllable frequency mediate the syllable effects, we conducted a short-term longitudinal study in French-speaking typically developing children who are learning to read (N = 50). We assumed that children early refer to orthographic regularities and sublexical orthographic and phonological frequency and structural properties of syllables to segment and access words. A lexical decision task in a visual masked priming paradigm was used to assess the automatic activation of syllables in good and poor readers at five different timepoints from Grade 1 to Grade 3. Our results confirmed a gradual use and reading level-dependent availability of phonological syllables. Good readers with good qualitative and quantitative phonological representations early automated and generalized the access and use of phonological syllables compared with poor readers who struggled with distributional and structural properties to access and use phonological syllables. Although high-frequency orthographic syllables consistently facilitated access to words in both good and poor readers, this was the phonological syllable frequency that modulated the expression, the direction, and the amplitude of the syllable effects, particularly in good readers. For the first time, our data provided a longitudinal overview of syllable effects at the first steps of learning to read in French-speaking children. These have implications for understanding reading acquisition and, by extension, set syllable-based reading interventions for children with reading difficulties.